FIRST    SCIENCE   BOOK 


PHYSICS  AND  CHEMISTRY 


BY 


LOTHROP  D.  BIGGINS,  Pn.B. 

INSTRUCTOR  IN  SCIENCE  IN  THE  STATE  NORMAL  SCHOOL 
DANBUBY,  CONN. 


GINN   &  COMPANY 

BOSTON  •  NEW  YOEK  •  CHICAGO  •  LONDON 


COPYKIGHT,   1905 

BY  LOTHROP  D.  HIGGINS 


ALL  BIGHTS  RESERVED 
514.11 


gfte 


GINN  &  COMPANY  •  PRO- 
PRIETORS •  BOSTON  •  U.S.A. 


PEEFACE 


This  book  is  designed  to  serve  as  an  introduction 
to  scientific  study,  and  at  the  same  time  to  present  a 
thorough  course  in  the  science  of  common  phenomena. 
Whether  the  pupil  has  been  prepared  by  courses  in 
"  nature  study  "  or  by  his  independent  observation  of 
things  about  him,  he  will  find  many  subjects  that  are 
already  known  to  him  here  treated  in  a  manner  which 
should  explain  the  mysteries  and  clarify  his  ideas. 
Finishing  this  course,  the  pupil  should  be  well  fitted 
to  take  up  the  science  studies  of  preparatory  schools, 
and  should  have  a  store  of  serviceable  knowledge. 

In  adapting  methods  and  language  to  the  view  point 
of  young  pupils,  the  author  has  drawn  upon  an  experi- 
ence of  several  years  with  them.  While  it  is  one  object 
of  the  book  to  teach  the  terms  and  expressions  of  science, 
care  has  been  used  to  keep  the  meaning  clear.  Methods 
of  treatment  that  easily  convey  wrong  impressions  have 
been  avoided,  as  well  as  those  which  offend  pupils  of 
grammar  school  age.  An  effort  has  been  made  to  show 
the  practical  bearing  of  the  various  subjects  upon  affairs 
in  our  daily  experience,  such  matters  being  introduced 
wherever  they  may  serve  to  illustrate  or  explain.  The 
experiments  are  simple  and  may  be  performed  by  the 
teacher,  or  by  the  pupils  with  his  oversight.  Briefly,  it 
is  believed  that  the  book  will  present  a  course  which 

iii 

374162 


iv  PREFACE 

shall  be  simple,  interesting,  and  instructive,  yet  losing 
nothing  of  its  accuracy. 

The  proof  has  been  read  by  Professor  G.  F.  Hull  of 
Dartmouth  College,  for  whose  critical  suggestions  the 
author  is  very  grateful.  Several  corporations  and  firms 
have  kindly  furnished  photographs  for  the  engravings  as 
follows  :  General  Electric  Company,  the  dynamo  ;  H.  K. 
Porter  Company,  Pittsburg,  Pennsylvania,  the  air  loco- 
motive ;  Baldwin  Locomotive  Works,  the  steam  locomo- 
tive ;  Illinois  Steel  Company,  the  blast  furnace ;  Cunard 
Steamship  Company,  the  steamship. 

LOTHROP  D.  HIGGINS 

CLINTON,  CONNECTICUT 
August,  1905 


CONTENTS 


PART  I.     PHYSICS 

CHAPTER  I 

MATTER  AND  ENERGY 

PAGES 
Definitions.     Scope  of  Physics.     Composition  and  States  of 

Matter.    Properties  of  Matter.    Gravitation.    Weight  and 
Specific  Gravity 1-22 


CHAPTER  II 
FLUID  PRESSURE 

Cause  of  Fluid  Pressure.  Pressure  in  Liquids.  Water  Supply. 
Buoyancy  and  Floating  Bodies.  Hydraulics.  Atmos- 
pheric Pressure.  Vacuum.  Barometer.  Pumps  a"nd 
Siphon.  Air  Pump.  Pressure  in  Gases.  Compressed 
Air.  Buoyancy  in  Gases 23-45 


CHAPTER  III 
MOTION  AND  FORCE 

Newton's  Laws  of  Motion.  Inertia.  Momentum.  Center  of 
Gravity.  Stability.  Centrifugal  Force.  Falling  Bodies. 
Pendulum.  Work  and  Power.  Machines  46-66 


CHAPTER  IV 
HEAT  AND  ENERGY 

Sources  of  Heat.     Explanation  of  Heat.    Temperature  and 
Thermometers.     Expansion  and  Contraction.     Changes 
of  State  due  to  Heat.    Evaporation  and  Condensation, 
v 


vi  CONTENTS 

PAGES 
Distillation.     Latent  Heat.     Conduction.     Convection. 

Kadiation  and  the  Ether.  Cooling  of  Bodies.  Artificial 
Cold.  Transformation  of  Energy.  Heat  as  a  Source  of 
Mechanical  Energy.  Heat  Engines  . 67-90 

CHAPTER  V 
SOUND 

Wave  Motion.  Vibration.  Sound  explained.  Sound  Waves. 
Echoes.  Forced  and  Sympathetic  Vibration.  Resonance. 
Tones  and  Noises.  Loudness.  Pitch.  Quality.  Voice  .  91-107 

CHAPTER  VI 
LIGHT 

Radiation  and  Light  Waves.  Luminous  and  Illuminated 
Bodies.  The  Ether.  Transparent,  Translucent,  and 
Opaque  Substances.  Shadows.  Reflection;  Mirrors. 
Refraction.  Prism  and  Lenses.  Formation  of  Images. 
The  Eye;  Camera;  Microscope;  Telescope.  Color. 
White  Light.  Spectrum.  Absorption.  Color  of  Objects.  108-128 


CHAPTER  VII 
ELECTRICITY 

Production  and  Control  of  Electrical  Energy.  Electrical 
Effects.  Potential  and  Electro-Motive  Force.  Elec- 
tric Charges.  Electrostatic  Induction.  Discharges. 
Lightning.  Electric  Current.  Voltaic  Cell.  Circuit. 
Resistance.  Batteries ;  Uses  of  Current.  Electrical 
Measurements.  Magnets.  Magnetic  Poles.  Magnetism 
of  the  Earth ;  Compass.  Induced  Currents.  Dynamo. 
Transformer.  Induction  Coil.  Uses  of  Electrical  Energy: 
Motor  ;  Cars  ;  Telephone  ;  Telegraph ;  Electroplating ; 
Lights  ............. 129-169 


CONTENTS  vii 


PAKT   II.     CHEMISTRY 

CHAPTER  VIII 

OUTLINE  OF  CHEMICAL  STUDY 

PAGES 
Scope  of  Chemistry.    Chemical  Changes.    Composition  and 

Decomposition.  Elements,  Compounds,  and  Mixtures. 
Atoms.  Chemical  Affinity.  Symbols.  Classes  of 
Substances :  Acids ;  Bases ;  Metals ;  Salts ;  Oxides ; 
Minerals;  Ores;  Alloys;  Solutions 171-191 


CHAPTER  IX 
COMMON  SUBSTANCES 

Elements:  Oxygen;  Hydrogen;  Nitrogen;  Carbon;  Sulphur; 
Phosphorus ;  Chlorine ;  Iron ;  Other  Metals.  Com- 
pounds: Water;  Sulphuric  Acid;  Carbon  Dioxide; 
Ammonia;  Cellulose;  Starch;  Sugars;  Alcohol;  Fats 
and  Oils ;  Soap.  Mixtures :  Air ;  Soil ;  Glass ;  Wood ; 
Paper;  Coal;  Illuminating  Gas;  Petroleum;  Explosives; 
Foods;  Fuels 192-220 


CHAPTER  X 
COMMON  CHEMICAL  PROCESSES 

Combustion.  Explosion.  Flame.  Oxidation.  Oxidation  in 
Animal  Bodies.  Decay.  Fermentation.  Bread  Making. 
Disinfection  .  .  221-230 


FIRST  SCIENCE   BOOK 
PART  I.    PHYSICS 

CHAPTER  I 

MATTER  AND  ENERGY 

SECTION  I 

DEFINITIONS 

1.  Introduction. — It  is  important,  in  beginning  a  new 
study,  to  have  some  clear  idea  of  what  the  study  is  to 
be.  The  word  science  may  perhaps  call  to  mind  strange 
things  of  which  we  have  heard,  so  that  we  think  of  it 
as  the  study  of  uncommon  things.  We  shall  find,  how- 
ever, that  science  treats  of  very  common  matters,  many 
of  which  we  already  understand  and  most  of  which 
affect  our  daily  lives.  The  main  difference  between 
scientific  study  and  simply  seeing  things  happen  is  a 
difference  in  the  manner  of  doing  it.  Scientific  study 
is  orderly;  each  fact  is  studied  in  connection  with 
others  that  are  like  it,  thus  making  the  whole  more 
simple.  In  science  we  are  not  content  to  find  that  a 
thing  does  happen,  but  we  try  to  find  out  how  it  hap- 
pens, what  causes  it,  and  what  is  its  effect  upon  other 

l 


2  MATTER  AND  ENERGY 

happenings  in  turn.  So  then  let  us  enter  upon  these 
studies  resolved  not  only  to  learn  all  that  we  can  but 
also  to  search  deeply  into  each  new  fact  until  we  fully 
understand  it. 

2.  Physics.  —  It  is  easy  to  see  that  if  science  is  the 
study  of  common  things,  it  must  include  a  great  number 
of  subjects  that  are  very  different  from  each  other.    In 
order  to  separate  these  subjects  so  that  each  may  be 
treated  more  simply,  scientific  study  is   divided  into 
many   branches,   such   as   physics,    chemistry,    botany, 
geology,  and  others.    Still  these  branches  are  more  or 
less  related  to  each  other,  the  teachings  of  one  often 
being  applied  to  several  of  the   others ;   in  fact,  one 
could  hardly  know  any  of  the  sciences  well  without 
knowing  something  about  one  or  more  besides.    The 
teachings  of  physics  are  perhaps  most  generally  used, 
and  for  this  reason  it  forms  a  natural  starting  point 
for  our  study. 

In  its  broadest  meaning,  physics  is  the  study  of  matter 
and  energy.  Concerning  matter,  physics  treats  of  such 
changes  as  affect  its  forms  and  motions. 

3.  Matter. —  To  understand  this  definition  it  is  neces- 
sary to  know  first  what  is  matter.    No  one  can  really 
tell  what  matter  is  —  it  can  only  be  described ;  and  it 
occurs  in  so  many  forms  and  has  such  various  features 
that  no  full  description  would  apply  to  all  sorts  of  mat- 
ter.   Some  kinds   are  of  one  color  and  some  another, 
while  many  show  no  color  at  all ;    different  kinds  vary 
in  weight;  there  are  hard  substances  and  soft;  and  in 
many  other  ways  we  look  in  vain  for  features  which 


DEFINITIONS  3 

shall  apply  to  all  bodies.  We  find,  however,  that  every- 
thing which  we  commonly  consider  to  be  matter  occu- 
pies space  or  takes  up  room.  Therefore, 
for  want  of  a  better  definition,  we  may 
say  that  matter  is  anything 
which  occupies  space. 


Experiment  1.  —  Hold  a 
tumbler  bottom  upward  and 
push  it  into  water,  as  in  Fig.  1. 
What  do  you  observe  ?  What 
is  in  the  tumbler  before  it  is 
pushed  downward?  Does  it 
take  up  room?  Give  a  rea-  p^  l 

son  for  your  answer.    Is  air 

a  sort  of  matter?  Is  there  any  form  of  matter  that  cannot 
be  seen? 

4.  Natural  Laws.  —  The  word  law  as  used  in  science 

» 

has  a  meaning  with  which  we  may  not  be  familiar.  A 
natural  law  is  simply  a  statement  of  a  happening  as  it 
occurs  in  nature.  Such  laws  are  not  made  by  man. 
From  time  to  time  men  may  find  out  new  ones  by 
studying  nature,  and  they  may  state  them  for  the  first 
time;  but  no  man  can  make  a  natural  law  or  destroy 
one.  Doubtless  there  are  many  natural  laws  which 
man  has  never  discovered,  though  we  can  see  the 
results  of  their  operation. 

5.  Energy.  —  This  is  a  subject  of  great  importance  in 
physics,  and  should  be  studied  carefully.    As  with  mat- 
ter, it  is  hard  to  say  just  what  energy  really  is ;  but  we 
may  say  for  a  definition :  Energy  is  the  ability  to  produce 
motion. 


4  MATTER  AND   ENERGY 

Matter  cannot  of  itself  cause  motion  ;  when  any  body 
of  matter  seems  to  cause  motion  in  itself  or  in  some 
other  body,  it  does  so  by  virtue  of  the  energy  which 
for  the  time  it  possesses.  A  body  of  matter  may  gain 
or  lose  energy  without  changing  in  size  or  weight, 
or  in  any  other  way  that  we  should  commonly  notice. 
Energy  must  then  be  something  quite  different  from 
matter,  having  no  substance  and  occupying  no  room, 
yet  very  important  because  without  it  no  motion  would 
be  possible. 

6.  Force.  —  When  the  energy  of  a  body  is  used  in  an 
effort  to  cause  motion,  we  generally  say  that  force  is 
exerted.  Thus  a  moving  body  is  said  to  "  exert  force  " 
upon  anything  that  it  strikes.  Similarly  in  the  case  of 
a  bat  acting  upon  a  ball,  a  bowstring  upon  an  arrow, 
a  hanging  body  upon  the  support  from  which  it  is  sus- 
pended, steam  in  an  engine,  or  an  exploding  powder 
charge  in  a  gun, —  each  is  an  example  of  some  force 
acting.  Notice  that  the  effect  of  these  forces  is  to  cause 
motion  or  to  make  an  effort  to  do  so.  A  moving  body 
gives  motion  to  the  body  that  it  strikes,  even  if  it  is 
only  the  air;  the  ball,  the  arrow,  and  the  bullet  in  the 
gun  move  when  forces  act  upon  them.  But  in  some 
cases,  as  the  hanging  body  or  the  steam  in  a  boiler, 
the  force  may  not  be  seen  to  cause  motion;  still  the 
steam  pushes  hard  upon  the  sides  of  the  boiler,  and  the 
hanging  body  tries  to  pull  its  support  downward.  In 
these  examples  force  is  exerted;  but  because  it  is  not 
great  enough  it  may  seem  to  cause  no  motion,  and  we 
say  that  it  tends  to  produce  motion. 


COMPOSITION  OF  MATTER  5 

From  our  study  we  shall  become  more  familiar  with 
the  use  of  this  word  than  any  definition  can  make  us, 
but  for  the  sake  of  a  definition  we  may  say,  Force  is 
the  direct  cause  that  tends  to  produce  motion  or  a  change 
of  motion. 

7.  Forms  of  Energy.  —  Like  matter,  energy  occurs  in 
many  different  forms.     Naturally,  too,  we  apply  names 
to  many  different  forces  acting  in  the  world.     We  shall 
learn  more  about  these  forms  of  energy  and  these  forces 
as  we  study  further. 

QUESTIONS 

1.  What  do  you  understand  the  term  science  to  mean  ? 

2.  Why  is  scientific  study  divided  into  many  branches  ?  Name 
several  of  these  branches.     How  are  they  related  ? 

3.  Define  physics.     What   is   included   in   the  word   matter? 
Name  some  forms  of  matter.     Can  you  name  anything  which  is 
not*matter  ? 

4.  What  is  a  natural  law?    How  are  natural  laws  discovered? 
Are  there  any  that  are  not  yet  known ?  Can  natural  laws  be  broken? 

5.  Define  energy.     Define  force.     Carefully  show  the  differ- 
ence between  them. 

6.  Gwe  examples  of  bodies  which  have  energy  but  exert  no 
force.     Give   examples   of   forces   tending   to   produce   motion, 
without  succeeding. 

SECTION   II 
COMPOSITION   OF  MATTER 

8.  Three  States  of  Matter.  —  Matter  occurs  in  three 
states  or  conditions  —  as  solids,  liquids,  and  gases. 

Solids  are  those  bodies  that  keep  the  same  size  and 
shape  unless  changed  "by  some  outside  force.  Glass,  wood, 


6  MATTER  AND  ENERGY 

iron,  cloth,  paper,  wax,  ice,  leather,  and  rock  are  ex- 
amples of  solid  substances. 

Liquids  keep  the  same  size,  but  change  their  shape 
according  to  the  vessel  in  which  they  are.  Water  is  the 
most  common  liquid  ;  others  are  alcohol,  benzine,  kero- 
sene, ether,  and  mercury. 

Crases  do  not  keep  either  their  size  or  shape,  but  expand 
without  limit.  For  this  reason  a  gas  cannot  be  kept 


FIG.  2 


pure  unless  it  is  in  a  tightly  closed  vessel.  If  a  bottle 
of  some  gas  be  uncorked  and  left  so,  in  a  few  minutes 
the  gas  will  escape  into  the  air  and  the  bottle  will  con- 
tain only  a  very  little,  mixed  with  a  large  amount  of 
air.  This  is  because  the  tiny  particles  of  any  gas  are 
always  in  rapid  motion,  and  so  they  become  separated 
from  each  other,  mixing  with  particles  of  other  gases. 
This  process  is  called  diffusion.  Gases  are  much  lighter 
than  liquids  or  solids.  Most  of  them  are  invisible  (cannot 


COMPOSITION  OF  MATTER  7 

be  seen)  and  only  a  few  have  any  color.  Air  is  a 
gas,  as  is  also  steam,  hydrogen,  oxygen,  chlorine,  and 
others.  Many  gases  which  cannot  be  seen  may  be  dis- 
covered by  their  odor ;  as  coal  gas,  illuminating  gas, 
ammonia,  etc. 

Vapor  is  a  name  given  to  such  gases  as  easily  change 
to  liquids ;  as  steam.  True  steam  is  invisible,  the 
white  cloud  that  we  call  steam  being  made  of  tiny 
drops  of  water.  At  the  spout  of  a  kettle  we  sometimes 
notice  a  seemingly  vacant  space  (Fig.  2)  where  there  is 
really  steam;  in  a  moment  this  steam  has  cooled  into 
drops.  Smoke  is  not  a  gas,  but  is  a  mass  of  solid 
particles.  • 

9.  Changes  of  State.  —  In  general,  substances  change 
from  solids  to  liquids  and  from  liquids  to  gases  upon 
being  heated.  For  some  kinds  of  matter  great  heat  is 
necessary. 

Experiment  2.  —  Gently  heat  small  quantities  of  ice,  wax,  par- 
affin, sugar,  or  butter,  and  note  the  changes  which  take  place. 

Experiment  3. — Fit 
a  stopper  into  a  test 
tube,  and  through  the 
stopper  run  a  glass 
tube,  as  in  Fig.  3. 
Into  the  test  tube  put 
a  small  quantity  of 
water,  alcohol,  ether, 
or  benzine.  Dip  the 

open  end  of  the  glass  FlG  3 

tube  under  water  and 

gently  heat  the  liquid  in  the  test  tube.  Note  carefully  and  explain 
all  that  you  see.  Be  sure  to  apply  only  slow  heat  and  use  caution 
with  volatile  liquids. 


8  MATTER  AND  ENERGY 

In  general,  gases  change  to  liquids  and  liquids  to 
solids  upon  being  cooled.  Air  and  other  gases  are  now 
changed  to  liquids,  and  some  even  to  solids,  by  cooling 
to  a  very  low  degree.  Steam  is  so  commonly  changed  to 
liquid  particles  that  we  carelessly  call  the  liquid  "  steam." 

Experiment  4.  —  Boil  some  water  in  a  test  tube.  Hold  a  cool 
dish  in  the  steam  just  as  it  escapes  from  the  tube.  (A  glass  dish, 
if  clean  and  dry,  may  best  serve  the  purpose.)  Explain  what  you 
observe. 

Experiment  5.  —  Melt  some  paraffin,  wax,  or  sugar,  and  let  it 
cool  rapidly.  What  change  takes  place  ?  Under  what  condition 
does  the  change  occur? 

Most  forms  of  matter  occur  commonly  in  only  one 
state,  because  the  temperatures  at  which  they  would 
change  are  unusually  high  or  low.  Some  substances, 
however,  are  common  in  two  states:  ice,  wax,  sugar, 
and  vaseline  easily  change  to  liquids ;  while  such  liquids 
as  alcohol,  ether,  naphtha,  and  chloroform  readily  change 
to  the  gaseous  state.  Water  commonly  occurs  in  all  three 
states ;  as  ice  (a  solid),  water  (a  liquid),  and  steam  (a  gas). 

10.  Composition  of  Matter ;  Molecules.  —  All  matter 
is  made  up  of  tiny  particles  called  molecules.  A  mole- 
cule is  the  smallest  particle  of  any  substance  which  can 
exist  alone  without  changing  its  nature.  From  this 
definition  it  is  clear  that  a  molecule  is  too  small  to 
be  seen;  for  the  smallest  bit  of  matter  which  could 
be  seen  would  be  capable  of  division  into  other  bits 
much  smaller.  Still  we  know  that  there  must  finally  be 
particles  so  small  that  they  can  no  longer  be  divided ; 
and  although  no  one  has  ever  seen  them,  we  can  give 
them  a  name  —  molecules. 


COMPOSITION  OF  MATTER  9 

11.  Molecular  Theory — Now  because  these  molecules 
are  too  small  to  be  discovered,  no  one  knows  exactly 
how  they  are  put  together  to  form  any  body  of  matter. 
There  are  several  like  cases  in  scientific  study,  when 
things  cannot  be  definitely  known  but  are  only  explained 
by  guess.  Such  guesses  are  not  rash,  however,  or  hasty; 
they  are  made  by  men  who  have  studied  and  thought 
deeply,  and  may  generally  be  taken  as  probably  true 
explanations.  An  explanation  of  this  sort  is  called  a 
theory.  The  explanation  of  the  composition  of  matter 
is  called  the  molecular  theory  ;  while  it  cannot  of  course 
be  proved  by  ordinary  methods  of  proof,  it  is  generally 
believed  by  scientists. 

The  molecular  theory  states  that  the  molecules  in  any 
body  are  separated  from  each  other  by  spaces  called 
pores.  These  pores  are  larger  than  the  molecules  them- 
selves, and  in  them  the  molecules  are  supposed  to  move 
rapidly  to^  and  fro.  The  rapid  to-and-fro  motion  is 
called  vibration. 

Experiment  6.  —  The  existence  of  pores  may  be  shown  by  dip- 
ping a  piece  of  gold  (one  of  the  densest  of  solids)  into  a  cup  of 
mercury  (quicksilver).  The  molecules  of  mercury  will  fill  the 
pores  between  the  molecules  of  gold.  (The  mercury  can  be 
removed  from  the  gold  by  dipping  into  nitric  acid.)  Similarly 
iron  will  take  water  into  its  pores. 

QUESTIONS 

1.  Name  the  three  states  of  matter.    Define  each.    Give  exam- 
ples of  each. 

2.  What  features  do  most  gases  have  in  common  ?    What  is 
meant  by  diffusion  ?    How  is  diffusion  caused  ?    What  is  a  vapor  ? 
What  is  smoke? 


10  MATTER  AND  ENERGY 

3.  How,  in  general,  may  the  state  of  a  substance  be  changed  ? 
Give  any  common  examples  of  changes  caused  by  heating ;  by 
cooling. 

4.  Can  steam  be  seen  ?    Why  does  steam  always  form  a  white 
cloud  when  set  free  in  the  air  ?    Of  what  is  that  cloud  composed  ? 

5.  Give  examples  of  substances  common  in  two  states.    Why 
are  not  all  substances  common  in  more  than  one  state  ? 

6.  What  is  a  molecule  ?    What  are  pores  ? 

7.  State  the  molecular  theory  in  your  own  words.    What  does 
it  attempt  to  explain  ?    Is  it  known  to  be  true  ? 

SECTION   III 
PROPERTIES   OF   MATTER 

12.  Common  Properties.  —  The   features   which    any 
form  of  matter  possesses  are  called  its  properties;  such, 
for  example,  as  its  color,  density,  hardness,  and  the  like. 
As  there  are  many  different  substances,  there  are  also 
many  different  properties.    Very  few  of  these  are  pos- 
sessed by  all  forms  of  matter,  though  several  are  com- 
mon to  many  substances.    We  shall  consider  only  a  few 
of  the  more  common  properties. 

13.  Impenetrability.  —  This  long  word  names  a  prop- 
erty which  is  common  to  all  matter,  —  that  of  completely 
occupying  the  space  in  which  it  is.    This  fact  is  some- 
times stated   as  follows:   No  two   bodies   can   occupy 
the  same  space  at  the  same  time.    No  body  of  matter 
can  enter  a  space  already  filled,  without  first  driving 
out  the  substance  which  fills  it.    Notice  that  we  say  the 
same  space;  this  does  not  mean  that  two  things  cannot 
be  in  the  same  dish,  for  example,  since  they  can  both 
be  in  a  dish  together  without  filling  the  same  space. 


PROPERTIES  OF  MATTER 


Experiment  7.  —  Hold  a  bottle,  mouth  downward,  over  watei 
and  push  it  downward  (Fig.  4).  Compare  the  height  of  the  watei 
in  the  bottle  with  that  around  it  outside.  Give  a  reason  for  this 
difference.  Now  tip  the  bottle  side  wise,  as  in  Fig.  5,  and  note  all 
that  happens.  What  takes  place  now  that  did  not  occur  when 


FIG.  4 


FIG.  5 


the  bottle  was  held  in  the  other  position  ?    How  do  you  explain 
the  difference  ? 

It  is  hard  to  fill  a  small-mouthed  bottle  with  a  thick  liquid 
like  oil  or  molasses,  because  of  the  bubbles  of  air  which'  must 
escape  as  the  liquid  enters. 

14.  Cohesion. —  Cohesion  is  the  force  which  holds  the 
molecules  of  a  body  together.  This  is  a  property  common 
to  solids  and  liquids ;  the  molecules  of  gases,  we  have 
learned,  do  not  cohere,  and  therefore  are  free  to  become 
widely  separated.  The  greater  the  cohesive  force  between 
the  molecules  of  a  substance,  the  more  the  substance 
resists  being  broken  or  pulled  apart.  When  cohesion  is 
great,  a  body  is  said  to  be  tough. 

Experiment  8.  —  Try  to  break  pieces  of  different  substances, 
e.g.  wood,  glass,  leather,  bone,  steel  wire  (knitting  needles),  iron 
wire,  copper  wire,  etc.  Make  a  list  of  these  in  the  order  of  their 
cohesive  force. 

Experiment  9 — Hold  a  drop  of  water  on  a  glass  rod.  Is 
there  anything  about  this  to  show  that  the  molecules  of  water 
cohere  ? 


MATTER  AND  ENERGY 


Broken  solids  cannot  be  mended  by  simply  pushing 
the  pieces  together,  because  the  molecules  cannot  be 
forced  near  enough  to  each  other.  Some  substances, 
such  as  iron,  may  be  heated  until  soft,  however,  and 
then  the  broken  ends  may  be  pounded  until  they  unite. 
This  process  is  called  welding. 

15.  Adhesion. — Adhesion  is  the  force  which  holds  the 
molecules  of  one  body  to  those  of  another.    Only  a  few  sub- 
stances have  this  property,  and  even  they  will  not  adhere 
to  many  others.    No  paste,  glue,  or  cement  will  stick  to 

A  everything;  each 

is  made  for  cer- 
tain substances. 

Experiment  10. 

—  Balance  a  piece  of 
glass,  a  (Fig.  6),  with 
weights  on  the  pan  b. 
Place  a  vessel  beneath 
a,  and  pour  water 
into  the  vessel  till  its 
surface  just  touches 
the  piece  of  glass.  Add  more  weights  to  the  pan  b  until  the  glass 
a  is  lifted  from  the  water.  Why  do  you  have  to  add  more  weights  ? 
How  much  more  do  you  add  ?  What  force  are  you  now  measuring  ? 

16.  Hardness.  —  A  hard  substance  is  one  in  which 
the  molecules  resist  any  change  of  position.     This  prop- 
erty of  course  applies  only  to  solids,  for  the  particles 
of  liquids  and  gases  move  about  freely. 

Experiment  11.  —  Using  pieces  of  wood,  glass,  iron,  copper, 
lead,  soap,  and  quartz,  try  to  scratch  each  with  the  others. 
Which  scratches  all  of  them,  and  which  scratches  none  ?  Arrange 
them  in  a  scale  according  to  hardness. 


FIG.  6 


PROPERTIES  OF  MATTER  13 

Articles  made  of  soft  iron  may  be  hardened  by  heat- 
ing  to  a  high  degree  and  plunging  at  once  into  water 
or  oil.  Springs  are  tempered  in  this  way,  after  being 
bent.  Hardened  iron  may  be  made  soft  by  heating 
and  then  slowly  cooling ;  this  is  called  annealing.  Try 
each  process. 

17.  Brittleness.  —  It  may  be  noticed  that  certain  sub- 
stances, though  they  are  hard,  are  easily  broken  by  a 
blow.    Glass,  for  example,  is  easily  broken  to  bits  by 
a  blow  from  a  hammer,  by  being  dropped  upon  a  hard 
surface,  or  sometimes  by  mere  pressure   of  the  hand. 
Some  knife  blades,  though  of  very  hard  steel,  may  be 
snapped  in  the   fingers.    Such  substances  as  are  thus 
easily  broken  by  a  blow  are  said  to  be  brittle. 

Experiment  12 Test  the  brittleness  of  various  substances,  by 

hammering  lightly,  by  dropping  them  upon  a  hard  surface,  or 
by  trying  to  snap  them  in  the  fingers.  Chalk,  pasteboard,  glass, 
iron  wire,  a  steel  needle,  copper  wire,  a  cracker,  a  bit  of  china, 
a  watch  spring,  or  a  wafer  may  serve  as  examples. 

18.  Malleability.  —  A  solid  substance  that  may  be 
hammered   into    thin   sheets   is    said    to   be   malleable. 
Several  of  the  metals  are  very  malleable  and  may  be 
rolled  into  sheets  thinner  than  tissue  paper.     Gold  is 
the  most  malleable  of  substances ;  it  can  be  hammered 
into  sheets  that  are  only  ^nnnnr  °^  an  ^nc^  thick.    We 
are  familiar  with  thin  sheets  of  this  metal  under  the 
name  "  gold  leaf."  . 

Experiment  13.  —  Hammer  some  bits  of  different  substances 
into  the  thinnest  sheets  you  can  make.  Try  lead,  iron,  tin,  cop- 
per, aluminium,  and  others 


14  MATTER  AND  ENERGY 

19.  Ductility Some  solid  substances  may  be  drawn 

out  into  small  wires ;  they  are  said  to  be  ductile.    In 
several  cases  a  ductile  substance  is  also  malleable,  but 
this  is  not  always  true;  a  body  whose  malleability  is 
great  may  have  small  ductility,  and  the  reverse  is  also 
true.    The  ductility  of  platinum  is  very  great,  for  it 
may  be  drawn  into  wires  that  are  only  Q-^-Q-Q  of  an  inch 
in  diameter. 

Experiment  14.  —  Heat  a  small  glass  tube  in  a  flame,  holding 
it  at  both  ends.  When  the  tube  is  well  heated  in  the  middle 
(i.e.  red  hot  and  soft),  remove  it  from  the  flame  and  quickly  draw 
the  ends  outward.  Examine  the  portion  that  was  heated. 

20.  Tenacity.  —  The  property  whereby  a  solid  body 
resists  being  pulled  in  pieces  is  called  tenacity.    This 
property  is  similar  to  cohesion,  though  not  so  broad  in 
its  meaning.    Steel  wire  has  great  tenacity,  a  very  small 
wire  being  able  to  support  a  heavy  mass. 

Experiment  15.  —  Take  a  strip  of  fresh  writing  paper  two 
inches  wide.  Fold  so  as  to  make  it  one  inch  wide  and  of  double 
thickness.  Grasp  one  end  firmly,  while  some  one  else  grasps  the 
other  end.  Now  both  pull  steadily  till  the  paper  breaks  in  two 
pieces.  What  do  you  conclude  with  regard  to  paper? 

21.  Porosity.  —  We  have  learned  that  all  bodies  of 
matter  are  composed  of  molecules  and  pores  or  spaces 
between   molecules.    The    size    of    these    pores    varies 
greatly  in  different  substances,  but  in  any  body  they 
are  much  larger  than  the  molecules.    Even   in  dense 
masses,  such  as  a  piece  of  lead  or  silver,  these  pores  are 
present,    though   the   mass    appears    to    have  no  such 
spaces  and  none  can  be  seen  through  a  microscope. 


PROPERTIES  OF  MATTER  15 

Some  substances,  though  the  spaces  between  their 
molecules  cannot  be  seen,  include  in  their  structure 
many  larger  spaces  that  can  be  seen.  Such  substances 
are  usually  made  of  many  fibers  or  cells  loosely  put 
together,  and  the  spaces  between  these  small  parts  are 
sometimes  called  pores  also.  A  sponge  or  a  piece  of 
blotting  paper  clearly  shows  this  structure.  A  body 
of  this  sort  is  generally  said  to  be  porous. 

22.  Compressibility. —  Given  a  bundle  of  loose  cot- 
ton, we  know  that  it  could  be  crowded  into  a  much 
smaller  bundle  ;    but  in  such  a  case  its  fibers  would 
be  much  nearer  together.    In  a  similar  manner  the  mol- 
ecules of  some  substances  may  be  crowded  nearer  to- 
gether, the  pores  becoming  smaller  and  the  whole  body 
losing  some  of  its  size.    The  property  whereby  a  body 
may  thus  be  crowded  into  a  smaller  space  is  called 
compressibility.    As  a  general  rule,  solids  and  liquids 
are  not  very  compressible ;  great  force  is  required  to 
crowd  their  molecules  nearer  together.    Gases,  however, 
have  great  compressibility;  their  molecules  under  ordi- 
nary pressure   are  widely  separated,  and  when  great 
force  is  used  they  may  be  driven  very  much  nearer 
together. 

23.  Elasticity Elasticity  is  that  property  by  which 

a  body  goes  back  to  its  former  size  and  shape  after  the 
force  which   changed  it  has   been  removed.    It  is  very 
important  to  notice  that  a  body  is  elastic  not  because  it 
may  be  bent  or  stretched,  but  because  it  goes  back  to 
its  former  state  as  soon  as  the  force  ceases  to  act.    Ivory 
is  very  elastic,   as  is  glass;   rubber  is  not  so  nearly 


16  MATTER  AND  ENERGY 

perfect  in  elasticity.  Gases  are  very  elastic  and  liquids 
as  well.  Substances  like  clay,  putty,  and  butter  are  said 
to  be  inelastic. 

Experiment  16.  —  Fill  a  football  or  bicycle  tire  with  air  under 
pressure.  Push  upon  its  surface  with  the  hand,  at  once  removing 
the  hand.  Is  any  dent  left  in  the  surface  ?  Is  air  elastic  ? 

Measure  a  coiled  spring.  Push  it  at  both  ends  so  as  to  shorten 
it ;  then  let  go  and  again  measure  the  spring.  Is  it  any  shorter  V 
Is  it  elastic  ? 

24.  Crystallization.  —  Many  substances  have  the  prop- 
erty of  forming  themselves  into  crystals  upon  chang- 
ing from  a  liquid  to  a  solid  state.  These  crystals  always 
have  a  definite  shape,  though  in  different  substances  the 
shapes  may  differ.  Quartz  crystals  are  commonly  seen 
in  rocks ;  also  garnets.  Diamonds,  rubies,  emeralds,  and 

other  gems  are  crystals  of 
rock.  Sugar  is  the  crystal- 
lized juice  of  cane  or  beet, 
snowflakes  are  crystals  of 
water,  and  many  salts  are 
crystals. 

Experiment  17.  —  Dissolve 
some  alum  or  sugar  in  warm 
water  until  no  more  can  be  made 
FlG  7  to  dissolve.    Hang  a  string  in 

the  water  (Fig.  7)  and  let  it 

stand  for  a  day.  Examine  it  from  time  to  time.  State  what  you 
observe  and  try  to  explain  it. 

Experiment  18 Melt  some  roll  sulphur  by  heating  and  allow 

it  to  cool  slowly.  When  hard,  examine  the  mass  and  tell  what 
you  find.  Compare  the  results  of  these  last  two  experiments. 
Compare  the  methods  used, 


PROPERTIES  OF  MATTER  17 

Some  substances  do  not  crystallize;  these  are  called 
amorphous  substances.  Crystalline  bodies  may  be  rec- 
ognized by  their  form,  or  sometimes  by  the  shining  sur- 
faces that  they  show  when  in  a  mass  together.  Butter, 
glass,  wood,  flour,  paper,  coal,  cloth,  and  wax  are  exam- 
ples of  amorphous  substances. 

25.  Capillarity.  —  The  adhesive  force  with  which 
some  liquids  are  attracted  to  certain  solid  substances 
causes  a  useful  and  interesting  action  called  capillary 
action  or  capillarity. 

Experiment  19.  —  Put  water  into  a  clean  glass  tumbler  and 
carefully  note  the  surface  of  the  water  where  it  meets  the 


Now  put  a  clean  glass  tube  of  very  small  bore  down  into  the 
water  vertically.  (An  old  thermometer  tube,  open  at  both  ends, 
may  be  used.)  Note  the  height  of  water  in  the  tube.  What  force 
holds  it  in  that  position? 

Water  has  so  great  an  adhesion  for  glass  .that  small 
amounts  of  it  may  be  raised  by  means  of  this  force. 
The  smaller  the  tube,  the  higher  that  small  amount  of 
water  will  rise  in  it.  The  oil  rises  through  a  lamp  wick 
by  capillarity,  the  wick  being  a  woven  mass  of  tiny 
fibers. 

26.  Inertia.  —  One  property  of  matter  which,  though 
very  passive,  is  of  great  importance  is  that  of  inertia. 
To  state  it  briefly,  inertia  means  the  complete  lack  of 
any  ability  of  matter  to  cause  or  to  change  motion.  No 
body  at  rest  can  start  itself  moving ;  some  force  must 
be  used,  and  then  the  body  is  started  gradually.  Also 
no  moving  body  can  stop  itself  or  in  any  way  change 


18  MATTER  AND  ENERGY 

its  motion ;  again  some  force  must  be  applied  in  some 
way. 

This  is  one  of  the  few  properties  that  are  common  to 
all  matter.    It  will  be  treated  more  fully  in  §§  56,  59. 

QUESTIONS 

1.  What  is  meant  by  the  properties  of  matter?    Name  as 
many  as  you  can.    Are  any  properties  common  to  all  forms  of 
matter  ? 

2.  What  is  meant  by  impenetrability?    Name  an  example  of 
its  effect. 

3.  Define  cohesion.    Do  the  molecules  of  liquids  cohere?    Do 
those  of   gases  ?    Explain  how  a  blacksmith  uses   cohesion  in 
welding  iron. 

4.  Define  adhesion.    Name  some  substances  that  adhere  to 
each  other.    Which  have  the  greater  adhesion  generally,  solids 
or  liquids  ? 

5.  Upon  what  does   hardness  depend?    How  may  iron   be 
hardened?    How  may  a  body  be  annealed? 

6.  Name  some  substances  that  are  brittle.    How  would  you 
test   the   brittleness   of   a   body  ?    Are   brittle   substances  ever 
hard? 

7.  What  is  meant  by  malleability?    Would  you   expect  a 
brittle  substance  to  be  also  malleable?    Name  some  bodies  of 
matter  that  you  think  are  malleable. 

8.  Name  some  substances  that  you  know  to  be  ductile.    How 
do  you  know  that  they  have  this  property?    Is  a  malleable  body 
necessarily  ductile  ? 

9.  How  would  you  test  the  tenacity  of  a  body  ? 

10.  What  is  commonly  meant  by  a  porous  body  ?    What  sorts 
of  bodies  are  porous  ? 

11.  What  class  of  substances  is  most  compressible?    Explain 
why  bodies  of  matter  may  be  thus  compressed. 

12.  Define  elasticity.    Is  air  elastic  ?    Is  water  elastic  ?    Name 
some  common  uses  of  elasticity. 


PROPERTIES  OF  MATTER:   GRAVITATION        19 

13.  Under  what  conditions  may  crystals  be  formed?    Name 
some  substances  that  crystallize.     Name  some  substances  that  do 
not  form  crystals.    What  are  such  substances  called  ? 

14.  Explain  the  cause  of  capillary  action.    Name  some  impor- 
tant use  that  is  made  of  capillarity. 

15.  Name  the  property  shown  by  the  substance  in  each  of  the 
following  cases :  a  watch  spring  in  unwinding  ;  a  blotting  paper 
in  absorbing  ink ;  a  stick  when  it  is  not  easily  broken ;  a  bit  of 
steel  that  can  scratch  glass;  the  air  forced  into  a  bicycle  tire,- 
vapor  in  the  air  when  it  forms  into  snowflakes ;  a  wire  when  it 
supports  a  heavy  weight. 

SECTION   IV 
PROPERTIES  OF  MATTER:   GRAVITATION 

27.  Gravitation.  —  We  already  know  three  facts :  that 
bodies  near  the  earth  fall  towards  it  if  they  are  free 
to  fall ;  that  all  bodies  on  the  earth  are  held  down'  by 
some  means  which  we  cannot  see ;  and  that  the  earth, 
moon,  and   planets    are    held   in   place   also  .by  some 
invisible  means.    It  is  clear  that  there  must  be  some 
great  force  doing  these  things,  and  we  call  this  force 
gravitation. 

28.  Gravity.  —  Not  only  do  these  things  occur,  but 
scientists  tell  us  that  every  body  of  matter  has  the 
power  of  attracting  every  other  body  —  not  only  solids 
but  liquids  and  gases  as  well.    The  only  reason  that 
they  do  not  succeed  in  drawing  together  is  that  the 
earth  draws  each  body  more  strongly,  thus  holding  each 
in  place.    The  force  exerted  by  the  earth  in  attract- 
ing and  holding  bodies  is  just  the  same  as  the  common 
force  exerted  by  all  matter,  that  is,  gravitation;  but 


20  MATTER  AND  ENERGY 

for  convenience  it  is  called  gravity  when  spoken  of  in 
connection  with  the  earth. 

It  is  hard  to  realize  the  importance  of  gravity  and 
the  part  that  it  plays  in  our  daily  lives.  Without  this 
force  nothing  .would  stay  on  earth  if  it  were  once  moved 
upward ;  a  ball  thrown  into  the  air  would  never  return  ; 
a  locomotive  would  have  no  weight  upon  the  rails ;  and 
we  could  not  even  walk. 

29.  Law  of  Gravitation.  —  Care   must   be   taken   to 
avoid  thinking  of  gravitation  as  magnetic  force.    The 
two  are  very  different;  for  while  magnetism  is  shown 
in  only  a  few  substances,  gravitation  is  a  common  prop- 
erty of  all  forms  of  matter  equally.    One  form  of  mat- 
ter can  exert  as  much  of  this  force  as  another  form,  and 
the  amount  which  any  body  can   exert  depends   only 
upon  the   quantity  of   matter  that  it   contains.    It  is 
because  the  earth  contains  so  much  matter  that  the 
force  of  gravity  is  so  strong. 

When  two  bodies  attract  each  other,  the  strength  of 
the  force  depends  upon  two  things  —  the  quantity  of 
matter  in  them  and  the  distance  between  their  centers. 
The  greater  the  amount  of  matter,  the  more  they  attract 
each  other;  the  greater  the  distance,  the  less  they 
attract.  These  two  facts  taken  together  make  up  the 
Laiv  of  Gravitation. 

30.  Weight.  —  Keeping   this   law   in    mind,   a   little 
thought  will  show  us  that  the  force  with  which  two 
bodies  at  the  same  place  will  be  drawn  towards  the  earth 
depends  only  upon  the  quantities  of  matter  in  them. 
That  is,  the  more  matter  a  body  contains,  the  stronger 


PROPERTIES  OF  MATTER:   GRAVITATION       21 


will  be  the  action  of  gravity  upon  it.  Thus  by  measur- 
ing the  force  with  which  gravity  pulls  a  body  we  can 
judge  of  the  amount  of  matter  in  it. 

The  weight  of  any  body  is  the  measure  of  the  force 
with  which  gravity  pulls  it.  This  may  be  found  by 
holding  the  body  suspended  by  some  A  E 

known  force.  Fig.  8  shows  a  common 
spring  balance  ;  anything  hung  upon  the 
hook  will  be  pulled  downward  until  the 
stretched  spring  exerts  as  much  force 
upon  it  as  does  gravity;  there  it  will 
stop,  and  the  pointer  will  show  this  force 
in  pounds  or  ounces,  etc. 

31.  Specific  Gravity.  —  Of  two  pieces 
of  lead,  the  larger  weighs  more ;  but  a 
piece  of  lead  may  weigh  more  than  a 
much  larger  piece  of  wood.  That  is,  some 
forms  of  matter  are  naturally  heavier  than  others.  In 
order  to  compare  the  weights  of  different  forms  of  matter, 
we  must  weigh  equal  amounts  of  volume  (size)  of  the  dif- 
ferent substances.  To  express  these  comparisons  easily, 
the  weight  of  each  substance  is  referred  to  that  of  water 
as  a  standard.  For  example,  a  piece  of  iron  is  found  to 
weigh  seven  times  as  much  as  an  equal  volume  of  water, 
a  piece  of  lead  eleven  times  as  much,  a  piece  of  gold 
nineteen  times  as  much,  and  so  on.  A  list  is  then  made, 
each  substance  being  named  and  followed  by  the  num- 
ber showing  how  many  times  the  substance  is  heavier 
than  water,  and  the  number  is  called  the  specific  gravity 
of  the  substance. 


FIG.  8 


22  MATTER  AND  ENERGY 

Specific  gravity  is  the  weight  of  a  substance  compared 
with  the  weight  of  an  equal  volume  of  water.  The 
specific  gravity  of  water  is  1 ;  of  cork,  0.2  ;  of  ice,  0.9  ; 
of  iron,  7.2  ;  of  lead,  11.4 ;  of  mercury,  13.6  ;  of  gold, 
19.4  ;  of  glass,  3.4 ;  and  of  platinum,  21.2. 

QUESTIONS 

1.  Name  some  examples  of  the  effect  of  gravitation. 

2.  What   is   meant   by   gravity?    Is    it   any   different   from 
gravitation  ? 

3.  Name  some  effects  of  gravity.    If  there  were  no  such  force, 
would  bodies  fall  to  the  ground  ?    Why  could  we  not  walk  if  there 
were  no  such  force  ? 

4.  Upon  what  two  things  does  the  strength  of  gravitation 
depend  ?    Would  the  moon  attract  a  body  more  or  less  than  the 
earth,  —  that  is,  would  a  body  weigh  more  on  the  moon  or  on 
the  earth?    If  we  were  on  the  moon,  could  we  jump  higher  than 
we  can  on  earth? 

5.  Define  weight.    Explain  how  weight  gives  us  an  idea  of  the 
amount  of  matter  in  a  body. 

6.  What  is  specific  gravity  ?    Does  it  depend  upon  the  kind  of 
matter  or  upon  the  size  of  the  body?    Would  a  lump  of  gold 
weigh  more  or  less  than  the  same  volume  of  lead? 


CHAPTER   II 
FLUID   PRESSURE 

SECTION   I 
PRESSURE   IN  LIQUIDS 

32.  Fluids.  —  Liquids  and  gases  are  called  fluids 
because  they  will  flow.  This  property  of  liquids  and 
gases  is  due  to  the  fact  that  their  molecules  move  freely 
among  each  other  from  place  to  place.  The  molecules  of 
a  solid,  of  course,  vibrate,  each  in  its  position  (§  11),  but 
none  of  them  can  easily  change  its  position  among  the 
others ;  hence  a  solid  body  preserves  its  shape.  The 
molecules  of  fluids,  on  the  other  hand,  change  their  posi- 
tion so  easily  that  the  simple  force  of  gravity  is  enough 
to  move  them,  pulling  each  downward  as  far  as  it  will  go. 

Experiment  20.  —  Pour  a  tumbler  of  water  into  a  large  flat  dish  ; 
it  spreads  out  to  the  edges  of  the  dish.  Pour  it  upon  a  larger  sur- 
face (a  board  or  piece  of  glass)  and  note  what  happens.  How 
may  this  be  explained  ?  What  force  acts  upon  the  liquid  ?  Is  its 
action  strongly  resisted? 

Thus  we  learn  why  it  is  that  liquids  flow  downward. 
Gravity  acts  in  the  same  way  upon  the  molecules  of 
solid  bodies  also,  pulling  each  of  them  downward;  but 
in  them  the  force  which  holds  each  molecule  in  its  place 
among  the  others  is  greater  than  the  force  of  gravity 
upon  it,  so  that  it  does  not  move.  In  liquids  and  gases 
the  molecules  are  free  to  move  as  gravity  pulls  them. 

23 


FLUID  PRESSURE 


33.  Cause  of  Pressure  in  Liquids.  —  If  a  liquid  flow 
or  be  poured  downward  until  it  is  stopped,   gravity 

still  acts  upon  it  and 
causes  it  to  push  upon 
whatever  stops  it. 
Thus  when  the  flow  of 
a  liquid  is  stopped  by  the 
bottom  and  sides  of  a  dish,  a 
pond,  or  even  the  ocean,  the 
liquid  exerts  force  upon  those 
sides  and  the  bottom.  Each 
particle  is  still  acted  upon 
by  gravity,  and  in  an  effort 
to  go  lower  it  exerts  force 
upon  the  particles  around  it 
in  all  directions.  For  this 
reason  water  will  spurt  from 
a  leaky  hose  equally  in  any 

direction.    This  principle  may  be  stated  as  follows  :  At 
the  same   depth  in  a  liquid,  pressure  is 
equal  in  all  directions. 


Experiment  21.  —  Cover  the  end  of  a  lamp 
chimney  with  cardboard  and  push  it  into  water 
in  a  glass  dish  (Fig.  9).  Pour  water  into  the 
chimney  till  it  reaches  the  same  level  as  the 
water  outside.  Add  a  bit  more  water  and  watch 
the  result.  Explain  what  you  see. 

Experiment  22.  —  Bend  three  tubes,  as  a,  b, 
and  c  (Fig.  10),  so  that  the  ends  may  open 
upward,  downward,  and  sidewise.  Put  equal 
quantities  of  mercury  into  each,  so  that  it  may 
stand  at  the  same  level  in  all.  Now  lower  the  tubes  into  water  till 


FIG.  10 


PRESSURE  IN  LIQUIDS  25 

the  openings  are  all  at  the  same  depth.  The  mercury  is  thus  forced 
up  into  the  long  arm  of  each  tube  (which  must  reach  above  water) 
by  the  force  of  the  water  at  the  end.  Compare  the  height  of  the 
three  mercury  columns.  What  does  this  show  regarding  liquid 
pressure  at  a  given  depth? 

34.  Pressure  depends  upon  Depth.  —  Now,  since  pres- 
sure in  liquids  is  caused  by  the  action  of  gravity  upon 
their  molecules,  and  gravity  acts  downward  only,  it  is 
clear  that  the  pressure  upon  any  point  will  depend  only 
upon  the  weight  of  the  molecules  above  it.    It  is  also 
clear  that  the  weight  will  depend  upon  the  number  of 
particles  above  the  point,  .and  that  their  number  will 
in  turn  depend  upon  the  depth  of  that  point  below  the 
surface.    From  this  we  may  state  the  general  principle : 
In  any  liquid,  pressure  upon  a  point  increases  with  its 
depth  below  the  surface. 

0 

Experiment  23.  —  Push  an  empty  can  into  water  slowly,  tak- 
ing care  not  to  get  it  entirely  below  the  surface.  Do  you  notice 
any  difference  in  the  force  that  you  have  to  exert  as  the  can  goes 
farther  down? 

35.  Surface  Level.  —  Since  gravity  pulls  all  particles 
of  a  liquid  as  low  as  possible,  and  the  particles  are  all 
free  to  move,  no  part  of  a  liquid  surface  can  be  higher 
than  another  unless  acted  upon  by  some  force  that  is 
stronger  than  gravity ;  that  is,  the  surface  of  a  liquid 
at  rest  is  always  level. 

36.  Water  Supply.  —  If  a  vessel  a  (Fig.  11)  be  filled 
with  water  to  a  height  cc'  and  then  connected  with  an 
empty  vessel  b  by  a  tube  at  the  bottom,  the  water  will 
flow  out  of  a  and  rise  in  b  until  it  stands  at  the  same 


26 


FLUID  PRESSURE 


level  in  both  vessels.  G-ravity  makes  the  liquid  flow  from 
a  and  causes  liquid  pressure  which  is  great  enough  to 

force  it  upward  into  b.  In 
the  same  way  these  forces 
are  used  to  give  cities  a 
supply  of  water. 

Pipes   from   a   pond   or 
"  reservoir,  located  on  high 
land,  lead  the  water  down 

into  the  city.  Gravity  of  course  causes  it  to  flow  down- 
ward, giving  it  pressure  enough  to  fill  the  pipes.  In 
these  pipes  the  water  may  rise  to  the  tops  of  buildings, 
provided  they  are  not  as  high  as-  the  surface  in  the 
reservoir  (see  Fig.  12).  These  pipes  may  be  tapped  at 
any  points  by  faucets,  hydrants,  or  fountains,  out  of 
which  the  water  will  run  with  some  force.  The  force 
with  which  the  water  runs  is  called  its  head;  the  head 
of  water  at  any  point  generally  increases  with  the  verti- 
cal distance  from  the  point  to  the  surface  in  the  reservoir, 


FIG.  12 


as  If  (Fig.  12).  Some  force  is  used  up  by  the  rubbing 
of  the  water  on  the  pipes  as  it  flows,  so  that  its  head  is 
less  as  the  distance  away  from  the  source  increases. 

37.  Buoyancy.  —  We   have    doubtless    noticed   that 
many  bodies  seem  to  weigh  less  when  held  in  water. 


PRESSURE  IN  LIQUIDS  27 

In  lifting  an  anchor  or  a  stone  from  under  water,  it 
seems  to  be  heavier  the  moment  it  rises  above  the  sur- 
face. It  may  be  said,  in  general,  that  all  bodies  seem  to 
weigh  less  when  held  in  a  liquid.  This  is  not  because 
the  thing  really  does  weigh  less,  but  because  it  is  then 
acted  upon  by  some  other  force  which  acts  in  the  opposite 
direction  to  the  force  of  gravity.  The  force  is  exerted 
by  the  liquid  body  and  is  called  buoyant  force. 

Experiment  24.  —  Hang  several  heavy  bodies  (e.g.  a  stone  or 
a  scrap  of  iron),  one  at  a  time,  to  a  sensitive  spring  balance. 
Note  the  weight  of  each.  Then,  without  removing  it  from  the 
balance,  lower  each  over  water  till  it  dips  wholly  below  the  sur- 
face, and  again  note  its  weight.  Does  it  pull  the  pointer  down 
more  or  less  when  in  the  water  ?  How  do  you  account  for  this  ? 
Does  each  body  really  change  in  weight  or  only  seem  to?  Can 
you  measure  the  buoyant  force  in  each  case? 

38.  Buoyant  Force  explained The  molecules  of  any 

liquid  at  rest  will,  of  course,  be  as  low  as  gravity  can 
pull  them.  If,  now,  any  body  be  lowered  into  that  liquid, 
some  of  the  particles  will  be  displaced  (pushed  out  of 
their  places)  and  the  surface  of  the  liquid  will  be  raised 
by  an  amount  just  equal  in  volume  to  the  size  of  the 
body  which  displaced  it.  These  particles  will,  of  course, 
be  still  pulled  downward  by  gravity,  and  in  tending  to 
return  to  their  places  they  will  exert  force  upon  the 
body.  Since  this  force  will  cause  greater  pressure  upon 
the  bottom  (see  §  34),  the  body  will,  of  course,  be  pushed 
upward. 

It  is  clear  that  any  body  held  entirely  in  a  liquid  will 
displace  its  own  volume  (size)  of  the  liquid.  And  as  the 
body  is  buoyed  upward  by  the  force  that  these  displaced 


FLUID  PRESSURE 


particles  exert,  it  follows  that  the  force  tending  to  hold 
it  up  is  equal  to  the  weight  of  the  displaced  liquid.  In 
other  words,  any  body  held  in  a  liquid  is  buoyed  up  by 
a  force  equal  to  the  weight  of  the  liquid  displaced. 

Experiment  25.  —  Fill  a  vessel  a  (Fig.  13)  with  water,  so  that 
no  more  can  be  added.  Weigh  some  heavy  body  in  air  and  again 
while  dipped  in  the  water,  as  in  Fig.  13.  Note  the  loss  of  weight. 


a  b 

FIG.  13 

Arrange  a  vessel  b  so  as  to  catch  the  water  which  spills  from  a 
as  r  is  lowered  into  it.  Weigh  the  water  caught,  comparing  with 
the  loss  of  weight  just  found. 

39.  Floating  Bodies.  —  The  specific  gravity  (see  §  31) 
of  many  substances  is  less  than  that  of  water;  such 
bodies  float  upon  its  surface.  If  any  body  floats  upon  a 
liquid  surface,  neither  rising  nor  falling,  it  is  clear  that 
its  weight  is  just  balanced  by  the  buoyant  force  acting 
upon  it.  But  we  have  learned  that  buoyant  force  is 
always  equal  to  the  weight  of  liquid  displaced.  From 
these  two  facts  we  can  easily  form  the  Law  of  Floating 
Bodies :  Floating  bodies  displace  an  amount  of  liquid 
equal  to  their  own  weight. 


PRESSURE  IN  LIQUIDS 


29 


Experiment  26.  —  Using  blocks  of  different  sorts  of  wood,  of 
cork,  ice,  Ivory  soap,  etc.,  float  each  upon  water.  In  each  case 
compare  the  amount  above  the  surface  with  that  below.  Try  to 
float  iron,  copper,  lead,  or  rock  upon  mercury. 

Many  heavy  substances  may  be  so  shaped  as  to  hold 
a  great  amount  of  air,  and  then  they  may  float.  Many 
vessels  are  now  made  of  iron  or  steel ;  they  float  because 
they  contain  so  much  space  filled  with  air  that  the 
vessel  as  a  whole  is  lighter  than  the  same  volume  of 
water. 

40.  Specific  Gravity  of  Liquids.  —  Like  solid  sub- 
stances, liquids  vary  much  in  the  kind  of  matter  of 
which  they  are  made,  and  therefore  they  differ  in  weight. 
Hence  it  is  desirable  to  know  the  specific 
gravities  of  liquids  as  well,  as  solids.  In 
this  case,  also,  water  is  used  as  the  stand- 
ard,  the  weight  of  the  various  liquid  sub- 
stances being  compared  with  that  of  an 
equal  volume  of  water.  To  avoid  weigh- 
ing the  liquid  a  simple  device  is  commonly 
used,  called  an  hydrometer.  An  hydrome- 
ter is  a  hollow  tube  of  glass  weighted  at 
one  end  and  having  a  scale  of  specific 
gravities  marked  on  its  stem  (Fig.  14). 
Upon  being  put  into  a  liquid  it  sinks  more 
or  less,  according  as  the  substance  is  light  or  heavy, 
and  the  mark  on  the  scale  where  the  liquid  surface 
rests  will  show  the  specific  gravity  of  that  substance. 

Experiment  27.  —  Shake  some  oil  and  water  together  in  a  test 
tube.  Let  it  stand  some  minutes;  examine  and  explain. 

Put  a  drop  of  mercury  into  a  glass  of  water.    What  happens  ? 


FIG.  14 


30 


FLUID  PRESSURE 


FIG.  15 


41.  Hydraulic  Press. —  The  use  of  liquid  bodies 
to  transmit  pressure  is  common  and  important. 
The  principle  may  be  stated  as  follows :  If  a  body 
of  liquid  be  entirely  closed  up  in  any  vessel,  and 
any  part  of  its  surface  be  put  under  pressure  from 
without,  that  pressure  will  be  felt  just  as  greatly 
upon  every  equal  part  of  the  inside  surface  of  the 
closed  vessel.  For  example,  the  inside  surface  of  a 
bottle  is  fifty  square  inches  and  the  lower  surface 
of  its  stopper  is  one  square  inch ;  if  the  bottle  is 
full  of  water,  the  stopper  fits  tightly,  and  a  force  of 
two  pounds  pushes  the  stopper 
down  upon  the  water,  and  every 
one  of  the  fifty  square  inches  of 
inside  surface  in  the  bottle  feels 
a  pressure  of  two  pounds  upon  it.  The  pressure  on  the 
whole  inside  surface  of  the  bottle  is  (50  x  2)= 100  pounds. 

Experiment  28.  —  Get  a  shallow  circular  pan,  make  a  small  hole 
in  its  side,  and 
solder  into 
this  a  short 
metal  tube 
(Fig.  15).  To 
this  tube  at- 
tach a  rub- 
ber tube  two 
or  more  feet 
long.  Tie  a 
piece  of  sheet 
rubber  very 
firmly  over 

the  top  of  the  pan.    Fill  the  whole  with  water,  keeping  the  tube 
full.    Raise  the  tube  as  high  as  possible.    See  how  great  a  weight 


FIG.  16 


PRESSURE  IN  LIQUIDS  31 

of  books  you  can  raise  up  from  the  rim  of  the  pan.    To  what  is 
the  pressure  due? 

Fig.  16  shows  how  this  principle  is  used  in  the 
hydraulic  press.  The  piston  p  is  small  and  the  cylinder 
c  is  many  times  larger ;  if  p  is  pushed  downward,  water 
in  the  pipe  transmits  its  pressure  to  the  lower  end  of  <?, 
and  c  is  pushed  upward  with  as  much  greater  force  than 
is  used  at  p  as  the  lower  surface  of  c  is  greater  than 
that  of  p.  Very  powerful  presses  and  hydraulic  jacks 
are  made  in  this  manner. 

QUESTIONS 

1.  What  substances  are  called  fluids?     Why?     What  force 
causes  fluids  to  flow  ?     Does  the  same  force  act  upon  the  particles 
of  solid  bodies?     Why  do  they  nof  flow? 

2.  Explain  how  pressure  in  liquids  is  caused.     At  the  same 
depth  in  a  liquid  what  is  true  of  pressures  in  different  directions  ? 

3.  Upon  what  does  the  greatness  of  liquid  pressure  depend? 
State  the  law  or  principle  in  regard  to  this. 

4.  What  is  true  of  the  surface  of  a  liquid  at  rest  ?    Explain 
why  this  is  so.     Did  you  ever  see  a  liquid  surface  which  was  not 
level?     What  made  it  so?     Was  it  at  rest? 

5.  Explain  the  method  of  supplying  cities  with  water.     Could 
the  reservoir  be  lower  than  the  city  ?     How  high  will  water  rise 
in  the  pipes  ?     Why  does  it  not  rise  as  high  as  the  surface  in  the 
reservoir  ? 

6.  Show  the  cause  of  buoyancy  in  liquids.     Does  it  act  upon 
all  bodies,  heavy  and  light?     State  anything  of  this  sort  that 
you  have  experienced. 

7.  Give  the  general  law  for  buoyant  force.     State  the  Law  of 
Floating  Bodies.     Why  do  steel  vessels  float? 

8.  How  is  the  specific  gravity  of  a  liquid  most  easily  found? 
Cream  is  made  of  oily  particles.    Why  does  it  rise  to  the  top  of 
milk?     Why  does  it  not  rise  more  rapidly? 


32 


FLUID  PRESSURE 


SECTION    II 
ATMOSPHERIC  PRESSURE 

42.  Cause  of  Atmospheric  Pressure.  —  The  atmosphere 
is  a  mixture  of  gases,  commonly  called  air.  It  is 
known  to  extend  many  miles  -above  us  (probably  over 
one  hundred),  though  the  greater  part  of  it  is  within  five 
or  six  miles  of  the  earth's  surface.  Now  we  have  found 
that  air  is  matter,  for  it  occupies  space  (§  3),  and  we 

know  that  all  mat- 
ter is  acted  upon 
by  gravity  ;  there- 
fore we  see  that  the 
atmosphere  must 
have  weight. 

Experiment  29.  —  If 

possible,  secure  some 
vessel  from  which  the 
air  may  be  removed 
and  kept  out  —  like  the 
globe  in  Fig.  17.  Re- 
move  the  air  by  using 
an  air  pump,  and  close 

the  stopcock  c.  Balance  the  globe  with  weights  on  the  scale  beam 
at  a.  Then  open  the  stopcock,  letting  air  into  the  globe  b  again. 
Is  the  globe  now  balanced  by  the  weights  at  a?  How  do  you 
account  for  this?  Add  weights  until  both  sides  balance  each 
other.  What  do  the  added  weights  represent? 

If  the  air  has  weight,  then,  however  little  it  may  be, 
the  total  amount  above  us  is  so  great  that  it  causes  a 
considerable  pressure  at  the  surface  of  the  earth.  So 
we  see  that  atmospheric  pressure  is  produced  in  just  the 


17 


ATMOSPHERIC  PRESSURE  33 

same  way  as  pressure  in  liquids,  —  by  the  simple  action 
of  gravity  upon  its  particles,  —  and  like  liquid  pressure, 
it  is  felt  equally  in  all  directions,  —  upward,  downward, 
and  sidewise. 

43.  Greatness  of  Atmospheric  Pressure.  —  The  pres- 
sure of  the  air  upon  objects  at  the  level  of  the  sea  is 
about  fifteen  pounds   on  every  square  inch  of  surface. 
On  mountains  there  is,  of  course,  less  air  above  the  sur- 
face and  the  pressure  is  less;  the  difference  is  easily 
noticed. 

This  pressure  seems  very  great  for  the  simple  weight 
of  air,  but  we  must  remember  that  it  reaches  far  above 
the  earth  —  many  miles.  Water  affords  that  amount 
of  pressure  at  a  depth  of  only  thirty-three  feet,  and 
mercury  at  thirty  inches. 

We  do  not  notice  this  weight  of  air  because  we  have 
always  lived  under  it;  moreover,  it  does  not  crush  us 
because  it  is  equal  on  all  sides  of  us,  even  entering  the ' 
body  in  the  lungs. 

44.  The  Vacuum.  —  No  body  is   crushed  by  atmos- 
pheric pressure  for  the  reason  just  given  (§  43) ;  that  is, 
it  is  felt  evenly  on  all  sides  —  and  inside  as  well  as  out. 
In  a  bottle,  for  example,  the  pressure  of  air  inside  is 
just   the  same  as  that    outside  ;    but   remove    the   air 
entirely  from  the  bottle,  and  we  have  then  an  unequal 
condition,  —  no  pressure  inside  to  balance  fifteen  pounds 
on  every  square  inch  outside.    Clearly  a  weak  bottle 
might  be  crushed  by  that  weight. 

Experiment  30.  —  Draw  some  of  the  air  from  a  small  bottle 
by  suction,  closing  its  mouth  with  your  tongue.  Describe  all 


34 


FLUID  PRESSURE 


that  you  observe.    Try  to  pull  the  bottle  off  the  tongue.    Try  to 
find  an  explanation  for  these  things. 

Experiment  31.  — Blow  into  a  paper  bag  until  it  is  well  filled 
out ;  then,  without  crushing  it,  open  the  end  a  little  way  so  that 
the  air  inside  may  be  under  equal  pressure  with  that  outside. 
Now  putting  it  to  the  lips,  draw  out  some  of  the  air  and  note 
what  happens  to  the  paper  bag.  Explain. 

In  these  cases  the  air  was  partly  removed.  A  space 
containing  no  air  or  other  matter  is  called  a  vacuum. 
A  space  from  which  the  air  has  been  partly  removed  is 
called  a  partial  vacuum ;  the  air  remaining  in  it  is  said 
to  be  rarefied.  In  practice,  no  perfect  vacuum  can  be 
produced,  but  air  has  been  rarefied  to  one-millionth  of 
its  usual  density. 

45.  Some  Effects  of  Atmospheric  Pressure.  —  To  under- 
stand the  effects  of  atmospheric  pressure,  it  must  be  kept 
in  mind  that  whenever  a  partial  vacuum  is 
formed  in  any  space,  the  pressure  within  the 
space  is  less  than  that  of  the  air  around  it; 
also  that  the  atmosphere,  being  a  fluid,  exerts 
force  equally  in  every  direction  and  can  push 
itself  into  any  opening,  whatever  its  shape 
or  size.  In  other  words,  wherever  a  partial 
vacuum  exists,  the  atmosphere  tends  to  enter  or 
to  force  something  else  in.  The  effort  of  air  to 
do  this  is  seen  in  many  common  happenings, 
some  of  them  useful  and  some  annoying. 

Experiment  32.  —  Dip  the  end  of  a  clean  straw  or 
other  tube  into  clean  water.  With  the  other  end  at 
the  lips,  draw  the  air  from  the  straw.  What  condition 
do  you  tend  to  cause  within  the  tube  ?  Do  you  succeed  in  causing 
that  condition  ?  Why  ?  Describe  and  explain  all  that  you  observe. 


FIG.  18 


ATMOSPHERIC  PRESSURE 


35 


FIG.  19 


Fig.  18  shows  this  same  principle  applied,  though  in  this 
case  the  vacuum  is  formed  in  the  tube  by  raising  the  piston  p. 
Explain  this. 

Experiment   33.  —  Fill   a   small-mouthed  bottle   with   water. 
When  full,  pour  out  the  water  and  note  that  it  comes  in  spurts. 
Explain.  Now  do  the  same  with  a  bak- 
ing-powder can.    Does  the  water  run 
out  in  spurts?  Explain  the  difference. 
In  pouring  any  liquid  from  a  ves- 
sel through  a  small  opening,  a  small 
amount  will  run  out ;  this  causes  a 
partial  vacuum  within  the  vessel,  into 
which  the  air  forces  itself  and  checks 
the  flow  of  liquid  while  passing  through 
the  opening.    The  spurting  flow  thus 
caused  is  more  marked  in  thicker  liq- 
uids, like  molasses  and  oil.    To  get 
a  steady  flow,  an  opening  or  vent  is 
sometimes  made  in  the  vessel  above  the  liquid  surface  ;  through, 
this  the  air  may  run  constantly,  as  the  liquid  runs  out  in  an  even 
stream  below. 

Experiment  34.  —  Fill  a  tumbler  with  water  and  cover  it  with  a 
piece  of  stiff  paper.    Holding  the  paper  in  place,  quickly  invert 
the  tumbler  and  hold  it  as  in  Fig.  19.    Why  does  not  the  water 
A  B       run  out  ?   Pull  down  one  corner  of 

the  paper,  still  holding  the  tum- 
bler inverted.  What  difference  do 
you  note?  How  do  you  account  for 
this? 

Experiment  35. — Into  a  U-shaped 
tube  pour  mercury,  as  in  B  (Fig.  20). 
Now  tip  the  tube  till  the  mercury 

comes  to  one  end,  and  cover  that  end  with  the  finger.  Keeping 
the  finger  tightly  over  the  end,  return  the  tube  to  the  position 
in  A.  Explain  what  you  observe. 

Many  other  simple  experiments  may  be  performed,  especially 
if  an  air  pump  is  used. 


FIG.  20 


36 


FLUID  PRESSURE 


46.  The  Barometer.  —  The  barometer  is  a  device  for 
measuring  the  pressure  of  the  atmosphere.  This  may 
be  done  by  allowing  the  atmosphere  to  hold  up  as  high 
a  column  of  mercury  as  it  will,  and  then  weighing  the 
mercury.  If  the  column  had  a  cross  section  of  just 

one  square  inch,  its  weight 
would  show  us,  in  pounds, 
the  pressure  of  the  air  upon 
one  square  inch. 

Experiment  36.  —  To  make 
a  barometer,  take  a  glass  tube 
about  32  inches  long,  closed  at 
one  end ;  fill  this  with  mercury, 
closing  the  open  end  with  the 
finger,  as  in  A  (Fig.  21).  Invert 
it  into  a  cup  of  mercury,  as  in  J5, 
being  careful  to  keep  the  end 
tightly  closed  until  it  is  under 
the  surface.  Now  remove  the 
finger  ;  a  little  mercury  runs  out 
into  the  cup,  leaving  a  column 
about  30  inches  long  which  is 
held  up  by  atmospheric  pressure  on  its  lower  end. 

Since  the  tube  was  full  and  the  mercury  in  falling  from  the  top 
allowed  no  air  to  enter,  it  is  clear  that  a  vacuum  is  formed  in  the 
tube  above  the  liquid.  Thus  there  is  no  pressure  on  its  upper 
end,  so  that  the  column  is  as  high  as  the  atmospheric  pressure 
can  force  it. 

As  the  air  varies  in  weight  from  day  to  day  it  pushes 
the  column  higher  or  lower.  Therefore  the  higher  the 
column  of  mercury,  the  heavier  the  air.  Changes  in 
atmospheric  pressure  often  attend  weather  changes,  so 
that  a  change  of  weather  may  sometimes  be  foretold  by 


PIG.  21 


ATMOSPHERIC  PRESSURE 


37 


those  who  use  the  barometer.  The  measure  of  atmos- 
pheric pressure  is  commonly  expressed  by  the  height 
(in  inches)  of  the  mercury  column. 

47.  Lifting  Pump.  —  Water,  being  much  lighter  than 
mercury,  can  be  held  to  a  height  of  over  thirty  feet  by 
atmospheric  pressure.  Clearly,  then,  water  may  be  raised 
from  a  well  thirty  feet  deep  by  the  force  of  the  air,  if 


FIG.  22 


FIG.  23 


we  can  only  arrange  to  cause  a  vacuum  in  the  upper 
end  of  the  pipe.  The  work  of  a  common  pump,  then, 
is  to  cause  a  vacuum  in  a  pipe  and  to  allow  the  water 
to  run  out  above  it.  Figs.  22  and  23  will  help  to 
explain  how  this  is  done. 

Fig.  22  shows  the  downstroke  of  a  piston  p  moving  in 
a  pump  barrel.  A  valve  v  swings  freely  on  a  hinge.  As 
the  piston  moves  down,  air  in  the  space  s  pushes  the 
valve  open  and  escapes  above  it.  If  now  the  piston  is 


38 


FLUID  PRESSURE 


raised  (by  pushing  down  the  handle  A),  air  pushing 
upon  v  from  above  closes  the  valve;  thus  no  air  can 
get  into  s  and  a  vacuum  is  formed  there.  To  nil  this 
vacuum,  atmospheric  pressure  upon  the  water  in  the 
well  pushes  it  into  the  pipe.  A  few  strokes  of  the 
piston  removes  the  air  entirely  from  pipe  and  pump, 
bringing  the  water  up  to  the  piston,  as  in  Fig.  23. 

Fig.  23  shows  the  piston  on  its  upstroke.  The  valve 
v  is  closed  by  the  water  above  it,  which  is  lifted  to  the 
spout  by  lowering  the  handle.  The  space  below  p 
tends  to  become  a  vacuum,  but  is  kept  full  of  water 
by  atmospheric  pressure  in  the  well,  as  explained. 

48.  Force  Pump The  lifting  pump  can  raise  water 

only  as  high  as  the  air  can  hold  it.  To  send  it  on  to  any 
distance,  force  has  to  be  exerted 
upon  the  water  by  the  pump.  A 
device  for  doing  this  is  called  a 
force  pump;  a  diagram  is  shown 
(Fig.  24)  to  explain  its  operation. 
In  Fig.  24,  p  is  a  solid  piston 
called  a  plunger;  it  has  no  valve. 
A  valve  a  opens  into,  and  a  valve 
c  out  from,  the  barrel  b.  As  the 
plunger  is  raised,  a  opens,  letting 
water  into  b.  Now  when  p  is 
pushed  downward,  force  is  ex- 
erted upon  the  water  in  6,  which 

causes  a  to  close  and  c  to  open.  Thus  the  water  is  sent 
to  the  pipe  e  under  whatever  pressure  is  given  it  by 
the  plunger. 


ATMOSPHERIC  PRESSURE 


39 


A  dome  d  contains  air.  At  each  downstroke  of  p 
water  forced  into  e  rises  a  little  way  into  d.  The  air 
in  d,  being  elastic,  drives  out  the  water  during  an  up- 
stroke of  p,  and  this  keeps  up  a  more  even  flow  in  the 
pipe  e.  The  dome  is  not  strictly  needed,  but  is  gener- 
ally used  on  force  pumps  to  make  the  stream  steady. 

Water  may  be  forced  any  distance  if  the  pump  is 
strong  enough  to  do  it,  though  great  force  may  have 
to  be  used.  Windmills  and  hot-air  engines  are  commonly 
used  to  fill  small  tanks,  while  city  water-supply  systems 
make  use  of  enormous  pumps  run  by  steam.  Fire  engines 
are  only  steam  force  pumps. 

49.  Siphon.  —  A  siphon  is  a  bent  tube  used  for  lifting 
fluids  quietly  from  one  vessel  to  another.  It  makes  use 
of  two  forces  — 
gravity  and  atmos- 
pheric pressure. 

Fig.  25  shows 
a  simple  siphon. 
The  bent  tube  abc 
is  filled  with  liquid 
from  the  vessel 
w,  and  allowed  to 
hang  so  that  the  end  c  is  lower  than  the  surface  of  the 
water  (x)  in  m.  In  this  position  gravity  acts  upon  the 
liquid  in  both  arms  of  the  tube,  but  more  strongly 
upon  be  than  bx.  because  c  is  lower  than  x ;  thus  the 
water  will  run  downward  out  of  be.  This  tends  to 
cause  a  vacuum  in  the  tube  at  5,  and  atmospheric 
pressure  forces  water  up  ab  to  fill  this  vacuum.  Of 


FIG.  25 


40  FLUID  PRESSURE 

course  this  keeps  the  tube  full,  and  the  water  will 
continue  to  run  as  long  as  the  vertical  distance  be  is 
greater  than  that  from  b  to  x. 

Experiment  37.  —  Dip  a  long  rubber  tube  into  water  until  it  is 
full.  Pinch  the  end  of  the  tube  tightly  to  close  it ;  draw  this  end 
out  of  the  water,  letting  it  hang  over  the  side  of  the  vessel. 
When  the  end  is  lower  than  the  liquid  surface  in  the  vessel  let  it 
go,  placing  a  dish  to  catch  the  water  which  runs.  Now  pinch  the 
tube  somewhere  along  its  length,  and  again  remove  the  pressure ; 
the  stream  ceases  when  the  tube  is  pinched.  Does  it  flow  #gain 
when  the  pressure  is  removed  ?  How  can  you  stop  the  flow 
entirely  ? 

Siphons  are  convenient  in  getting  liquids  from  barrels  or 
other  vessels  from  which  they  cannot  be  easily  poured. 

QUESTIONS 

1.  What  is  the  atmosphere  ?    Is  it  matter  ?    Why  does  it  exert 
force  upon  objects  on  earth?    In  what  directions  is  that  force 
felt? 

2.  How  great  is  atmospheric  pressure?    Why  do  we  not  feel 
the  weight  of  it  ?    Why  are  not  some  bodies  crushed  by  it  ?    Is 
this  pressure  greater  or  less  upon  high  land  ?    Why  ? 

3.  What  is  a  vacuum?    What  is  a  partial  vacuum?    What  is 
the  condition  of  the  air  in  a  partial  vacuum  ?    Name  some  exam- 
ples of  vacua  which  you  have  observed. 

4.  How  does  the  atmosphere  behave  toward  a  vacuum  ?    State 
any  familiar  examples  of  this.    Why  does  a  liquid  run  from  a 
jug  in  spurts?     Why  does  it   not  run  in  the  same  way  from 
a  pitcher? 

5.  What  is  a  barometer?    How  is  it  made?    Would  a  tube 
serve  the  purpose  if  open  at  its  upper  end?    Why?    How  do 
barometer  changes  indicate  changes  of  weather? 

6.  Explain  how  atmospheric  pressure  is  used  in  raising  water 
from  wells.    How   high  may  water  be   raised  by   atmospheric 
pressure  ? 


PRESSURE  IN   GASES  41 

7.  What  is  the  use  of  a  lifting  pump?    Explain  its  action. 
How    does    a   force    pump    differ    from    a   lifting   pump   in  its 
action  ?    Of  what  different  use  is  it  ?    Explain  the  use  of  the 
dome. 

8.  Explain  the  action  of  the  siphon.    What  forces  are  used  by 
it  ?    How  is  the  flow  stopped  ? 

9.  Would  a  lifting  pump  serve  its  purpose  if  the  piston  did 
not  fit  tightly  in  the  pipe  ?   Why? 

SECTION   III 
PRESSURE   IN   GASES 

50.  Expansion  of  Gases.  —  Gases  differ  from  liquids 
and  solids  in  that  their  molecules  are  not  kept  near 
together  by  cohesive  force  (§  14).  Therefore,  since 
their  molecules  are  always  in  rapid  motion,  there  is 
no  force  exerted  by  the  gaseous  particles  to  prevent 
their  becoming  widely  separated.  Thus  if  a  bottle  of 
some  gas  be  left  open  in  a  room,  its  molecules  soon  mix 
with  the  air  and  move  to  all  parts  of  the  room.  Open 
the  bottle  of  gas  in  a  large  vacuum,  and  the  same  thing 
happens  ;  the  molecules  do  not  increase  in  size,  but  the 
spaces  between  them  increase  greatly.  This  increase  in 
the  volume  of  a  gaseous  body  by  the  wider  separation 
of  its  molecules  is  called  expansion. 

Gases  therefore  may  be  said  to  tend  always  to  ex- 
pand; and  as  their  molecules  exert  no  cohesive  force 
to  oppose  this  expansion,  a  gas  can  be  kept  in  a  certain 
space  only  by  inclosing  it  completely  within  walls  that 
may  supply  the  necessary  force.  If  a  vessel  is  filled 
with  a  gas  under  ordinary  pressure  of  the  air,  the  gas 
is  said  to  be  under  a  pressure  of  one  atmosphere.  If, 


42 


FLUID  PRESSURE 


now,  this  same  body  of  gas  expands  so  that  its  molecules 
are  farther  apart,  it  is  said  to  be  rarefied. 

Experiment  38 —  If  an  air  pump  can  be  had,  any  experiments 
like  the  following  will  serve  to  explain  the  point.  Into  the  soft 
bladder  of  a  football  allow  a  small  amount  of  air  to  enter  —  not 
enough  to  fill  the  ball,  by  any  means.  Close  the  opening  tightly 
and,  putting  it  under  the  receiver  of  an  air  pump,  remove  the  air 
from  around  it.  Watch  the  football  closely  while  this  is  being 
done.  What  change  occurs  in  the  air  within  the  ball  ?  How  is 
this  change  made  possible?  When  you  can  remove  no  more  air, 
note  the  appearance  of  the  ball  and  admit  the  air  again  to  the 
receiver.  Explain  what  now  occurs. 

51.  The  Air  Pump.  —  A  device  for  rarefying  gases  is 
called  an  air  pump.  Fig.  26  shows  a  common  sort.  Its 

action  is  similar 
to  that  of  the 
lifting  pump, 
except  that  the 
air  is  sent  from 
the  receiver  r  to 
the  pump  barrel 
c  by  its  own  ex- 
pansive force. 
As  each  stroke 
of  the  piston  p  removes  some  air,  the  expansive  force  of 
that  which  remains  grows  less  and  less  until  it  is  no 
longer  strong  enough  to  open  the  valves  a  and  c.  No 
more  air  can  then  be  removed,  and  the  vacuum  in  r 
will  not  be  perfect. 

A  newer  form,  the  mercury  air  pump,  has  no  valves 
to  be  moved  by  the  gas,  so  that  the  vacuum  formed 


/                         \ 

r 

• 
*jp  M 

-•  .-.  «  ^ 

FIG.  26 


PRESSURE  IN  GASES  43 

may  be  more  nearly  perfect.    With  this  pump  gases  may 
be  rarefied  to  one-millionth  of  one  atmosphere  (§  50). 

52.  Compression.  —  When  a  force  greater  than  the 
pressure  of  the  atmosphere  is^  exerted  upon  any  body 
its  molecules  may  be   crowded  nearer  together; 
the  substance  is  then  said  to  be  compressed.    In 
general,  solids  bear  almost  no  compression,  and 
liquids  only  a  little ;  but  gases,  whose  molecules 
are  commonly  far  apart,  may  be  compressed  into 
a  small  fraction  of  their  usual  volume. 


— i 


53.  Compressed  Air.  —  We  have  learned  that 
gases    are    elastic    (§  23) ;    moreover,    they   are 
perfectly  elastic.    That  is,  when  force  has  been 
used  to  compress  a  gas,  the  gas  will  exert  the 
same  amount  of  force  in  trying  to  return  to  its 
former  volume.    It  is  owing  to  this  fact  that        j  [ 
compressed  air  is  so  much  used  as  a  motive  force. 
Energy  may  be  stored  by  forcing  air  into  strong 
tanks  under  heavy  pressure  ;  the  tanks  are  then  carried 
about,  and  work  may  be  done  by  the  force  which  the 
air  exerts  when  it  is  allowed  to  escape.    Compressed-air 
engines  are  run  by  this  means. 

Experiment  39.  —  Fill  a  bicycle  tire  with  air  by  means  of  a 

a  ft      cycle  pump   (Fig.  27).    Does   it 

— -Q —  |,'^    become  harder  to  work  the  pump 

'    as  the  tire  becomes  filled  ?  Why  ? 

2g  Press  upon  the  tire  from  time 

to  time  with  the  finger.    Does  it 

become  harder  to  dent  the  tire  ?    Is  the  tire  more  strongly  elastic 
when  well  filled  ? 

Do  the  same  things  with  a  rubber  football. 


44  FLUID  PRESSURE 

Experiment  40.  —  Make  a  common  popgun,  using  a  piece  of 

elder  (removing  the  pith)  and  two  good  cork  stoppers.    Fit  the  stop- 

a  b  pers  as  in  Fig.  28,  and  push 

— — i — -=i  upon  the  one  at  b  until  a 

flies  out.    Explain.    How 
FIG.  29 

much  air  is  in  c  (Fig.  29) 

as  compared  with  c  (Fig.  28)  ?  What  is  its  condition  in  c  (Fig.  29)  ? 

54.  Buoyancy  in  Gases.  —  Gases,  like  solids  and 
liquids,  vary  much  in  specific  gravity.  If  it  were  not 
for  their  tendency  to  diffuse,  all  heavy  gases  would 
sink  to  the  ground  and  all  that  are  lighter  than  air 
would  rise.  In  a  general  way,  gases  do  this,  but  of 
course  they  soon  diffuse  and  lose  their  purity.  If  a  gas 
can,  however,  be  kept  in  a  very  light  covering  (e.g.  a 
soap  bubble),  it  will  rise  or  fall  in  the  air,  according  as 
it  is  lighter  or  heavier  than  air.  Thus  large  amounts  of 
hydrogen  gas  (only  -^  as  heavy  as  air)  may  be  put  into 
a  silk  covering,  and  the  whole  will  be  so  much  lighter 
than  the  air  that  it  will  rise.  In  this  way  balloons  are 
made.  When  large  enough  they  may  carry  up  heavy 
loads;  but  since  the  air  becomes  rarer  as  we  go  up 
from  the  earth,  there  is  a  limit  to  the  height  that  a 
balloon  may  reach. 

QUESTIONS 

1.  What  shape  does  a  liquid  body  assume  when  left  to  itself? 
(Think  of  liquids  that  are  freely  falling.)    What  becomes  of  a 
gas  when  left  free  in  space  ?    Explain  the  difference. 

2.  What  is  meant  by  expansion  ?    Give  examples. 

3.  What  is  meant  by  a  pressure  of  one  atmosphere  ?    What  is 
a  rarefied  gas  ? 

4.  What  is  an  air  pump?  Explain  its  action.    By  what  force  is  the 
gas  removed  from  the  receiver?  Can  it  be  entirely  removed?  Why? 


PRESSURE  IN  GASES  45 

5.  When  a  substance  is  compressed  what  happens  to  its  mole- 
cules ?    What  sort  of  matter  can  bear  most  compression  ?    Why  ? 

6.  Explain  why  compressed  gases  exert  force.    Name  any  uses 
of  compressed  air  that  you  know  of.    Is  a  hollow  rubber  ball  more 
elastic  with  or  without  a  hole  punched  through  it  ?     Why  ? 

7.  Why  do  balloons  rise  into  the  air  ?    How  does  the  weight 
of  a  balloon  compare  with  that  of  the  volume  of  air  that  it 
displaces  ? 

8.  Why  cannot  a  balloon  rise  to  unlimited  altitudes  ?    Which 
could  rise  higher,  a  balloon  filled  with  hydrogen  or  another  filled 
with  hot  air?    Why? 

9.  Explain  the  use  of  compressed  air  in  a  bicycle  tire. 


CHAPTER   III 
MOTION  AND   FORCE 

SECTION   I 
NEWTON'S    THREE  LAWS    OF   MOTION 

55.  Newton's   Laws.  —  Three    Laws    of  Motion   are 
named  from  Sir  Isaac  Newton,  an  English  philosopher 
who  was  the  first  to  state  them.    At  first  thought  they 
may  seem  strange ;  and  for  this  reason,  as  well  as  their 
great  importance,  they  should  be  studied  carefully  and 
committed  to  memory. 

First  Law :  A  body  at  rest  will  stay  at  rest,  and  a  body 
in  motion  will  keep  moving  in  a  straight  line  with  the  same 
speed,  unless  acted  upon  by  some  force. 

Second  Law:  A  change  of  motion  follows  the  direction 
of  the  force  which  causes  it,  and  is  proportional  to  the  amount 
of  force  used  and  the  time  during  which  it  acts. 

Third  Law :  To  every  action  there  is  an  equal  reaction 
in  the  opposite  direction. 

56.  The  First  Law No  doubt  we  can  at  once  call 

to  mind  several  cases  which  seem  to  prove  this  law 
untrue — but  think  a  moment.     Do  any  bodies  really 
begin  to  move  from  a  state  of  rest  without  the  action 
of   some   force   upon  them  ?     Can   any   moving   body 
actually  stop  of  itself? 

You  may  say  that  a  body  will  fall  to  the  ground  all 
of  itself ;  but  would  it  fall  if  the  force  of  gravity  did 

46 


NEWTON'S  THREE  LAWS  OF  MOTION  47 

not  act  ?  Will  an  engine  begin  to  run  without  water  in 
its  boiler  and  heat  under  it ;  or  an  electric  motor  without 
its  current  of  electricity?  Can  even  an  animal  move 
itself  without  the  energy  supplied  by  food  and  air  ?  In 
every  case  we  should  find  that,  if  we  knew  enough  about 
it,  we  could  trace  any  motion  to  some  outside  cause. 

Nor  is  it  any  easier  to  find  a  moving  body  which  stops 
without  force  being  used.  Many  bodies  may  seem  to  do 
so;  but  is  it  not,  after  all,  the  force  of  gravity  which 
stops  a  rolling  ball,  a  bullet,  or  other  such  moving 
body?  Unfortunately  we  cannot,  upon  earth,  find  ex- 
amples of  constant  motion  without  the  action  of  force, 
because  all  motion  (except  downward)  will  be  stopped 
by  gravity  if  not  by  other  forces ;  but  doubtless  many 
stars  and  planets  are  in  constant  motion  simply  because 
there  is  no  force  to  stop  them. 

After  all,  this  law  merely  states  that  no  body  of  itself 
can  alter  its  state  of  rest  or  motion,  and  that  is  not  very 
odd.  It  would  be  far  more  strange  if  things  could  start 
or  stop  their  own  motion  without  force  being  exerted. 
This  helplessness  of  matter  is  called  inertia. 

57.  The  Second  Law.  —  The  first  part  of  this  law 
(§  55)  may  easily  be  understood  —  any  moving  body 
will  go  in  the  same  direction  that  the  force  takes. 
Strike  a  nail  with  a  hammer  and  the  nail  moves  on  as 
the  hammer  was  moving.  If  two  or  more  forces  act 
upon  a  body  at  the  same  time,  the  effect  of  each  force 
appears  in  the  resulting  motion. 

Experiment  41.  —  At  the  same  instant  strike  a  ball  a  (Fig.  30) 
with  two  mallets  in  two  directions,  ab  and  ac.  One  blow  alone 
would  carry  it  to  the  right  as  far  as  6j  the  other  alone  would  send 


48 


MOTION  AND  FORCE 


it  downward  as  far  as  c.    The  ball  really  moves  along  the  line 
ad  to  d;  but  as  d  is  as  far  to  the  right  of  a  as  is  6,  and  as  far 

below  as  is  c,  each  force  has  had 

its  effect. 

The  second  part  of  the 
law  means  simply  that  the 
greater  the  amount  of  force 
used  or  the  longer  the  time 
that  a  force  acts,  the  greater 
will  be  the  amount  of  mo- 
tion caused. 


FIG.  30 


58.   The   Third  Law. — 

This  law  means  simply  that 
whenever  any  body  exerts 
force  upon  another,  the 
second  body  in  resisting 
that  action  exerts  the  same  amount  of  force  upon  the 
first.  If  we  strike  a  piece  of  wood  with  a  hammer, 
the  hammer  is  stopped  by  the  wood ;  clearly,  the  wood 
exerts  force  upon  the  hammer  in  stopping  it.  This 
force  is  just  equal  to  that  exerted  by  the  hammer, 
for  had  it  been  less,  the  motion  would  not  have  been 
stopped  entirely;  or  had  it  been  more,  the  hammer 
would  have  been  driven  back.  Force  exerted  in  this 
way,  being  caused  by  the  action  of  another  force,  is 
called  reaction.  Examples  of  reaction  are  common:  a 
boat  exerts  force  upon  the  water  as  it  moves,  and  the 
water  reacts  upon  the  boat,  tending  to  stop  it;  in  the 
same  way  the  air  reacts  upon  a  train  or  any  moving 
object;  it  is  well  known  that  a  bicycle  rider  can  go 
faster  if  he  follows  a  moving  shield.  In  all  these  cases 


NEWTON'S  THREE  LAWS   OF  MOTION  49 

note  that  there  would  be  no  reaction  if  there  were  not 
first  some  action. 

Reaction  is  often  very  useful.  Fig.  31  shows  a 
common  wood  screw,  A.  As 
the  screw  is  turned  around,  the 
threads  push  backward  upon 
the  wood  on  the  surfaces  a,  Fig. 
Z?;  the  wood  then  reacts  upon 
the  threads,  driving  the  screw 
forward.  Many  steamships  use 
screw  propellers.  These,  as  they 
turn  in  the  water,  exert  force 
backward  upon  it ;  then  the  reaction  of  the  water  upon 
them  drives  the  boat  forward. 


QUESTIONS 

1.  State  Newton's  three  Laws  of  Motion.    Tell  all  that  you 
know  about  Newton.    Try  to  find  out  a  little  more. 

2.  Give  any  examples  of  bodies  that  seem  to  set  themselves 
in  motion,  and  then  tell  what  outside  force  moves  them.    Why  do 
we  not  find  on  earth  any  examples  of  constant  motion  without 
force  being  applied? 

3.  If  two  equal  forces  should  act  upon  a  body  in  opposite 
directions,  what  would  be  the  result  ?    If  the  forces  were  unequal, 
what  would  be  the  result  ? 

4.  What  is   meant  by  reaction?    Could   there   be  any  reac- 
tion if  there  were  no  action?    Is  there  ever  an  action  without 
reaction  ? 

5.  Give  examples  of  reaction.    Explain  some  of  its  uses.    Show 
how  a  screw  propeller  drives  a  boat. 

6.  If  you  strike  a  wall  with  your  fist,  you  feel  pain.    Why? 
Why  does  it  not  give  equal  pain  if  you  strike  a  pillow  with 
your  fist? 


50  MOTION  AND  FORCE 

SECTION   II 
SOME  EFFECTS   OF   NEWTON'S   LAWS 

59.  Inertia.  —  The  tendency  of  a  body  to  remain  in 
its  state  of  rest  or  motion  has  been  called  inertia  (§  56). 
Owing  to  its  inertia,  a  body  acted  upon  by  force  gener- 
ally starts  slowly,  increasing  its  speed  as  long  as  the 
same  amount  of  force  acts.    We  have  often  received  a 
heavy  jarring  as  a  car  started  violently  from  a  state  of 
rest ;  this  is  because  the  back  of  the  seat  runs  into  us 
before  the  body  has  begun  to  move. 

Experiment  42.  —  Balance  a  visiting  card  on  the  end  of  the 
finger  and  place  a  coin  upon  it,  directly  above  the  finger  tip. 
With  the  other  hand  suddenly  snap  the  card  away  edgewise. 
After  a  bit  of  practice,  this  may  be  done  so  as  to  leave  the  coin 
upon  the  finger.  Why  does  not  the  coin  move  off  with  the  card? 
Slowly  push  the  card  off  the  finger.  Note  and  explain  any  dif- 
ference in  the  behavior  of  the  coin. 

60.  Momentum Inertia  also  causes  moving  masses 

to  continue  in  motion.    But  as  all  moving  bodies  on 
earth  are  verjr  soon  acted  upon  by  at  least  one  force 
tending  to  stop  them,  it  is  clear  that  the  ability  of  any 
body  to  keep  on  moving  will  depend  upon  its  ability  to 
overcome  opposing  forces.    And  this  in  turn  depends' 
upon  what   may  be   called  its  "  quantity  of  motion " 
or  momentum.    A  thrown  ball,  for  example,  is  set  in 
motion  by  force   exerted  upon  it  at  the  hand;   once 
out  of  the  hand,  its  progress  depends  upon  its  momen- 
tum, that  is,  the  quantity  of  motion  given  to  it  by 
the  arm. 


SOME  EFFECTS  OF  NEWTON'S  LAWS  51 

Experiment  43.  —  Using  the  same  ball,  roll  it  twice  over  the 
same  surface,  once  slowly  and  once  with  speed.  Note  the  dis- 
tances that  it  travels. 

Experiment  44.  —  Now  take  two  balls,  one  very  much  heavier 
than  the  other  (e.g.  a  tennis  ball  and  a  bowling  ball)  ;  roll  them 
over  the  same  surface,  starting  them  at  the  same  speed,  if  pos- 
sible. Note  the  distances  traveled. 

From  these  two  experiments  we  see  that  the  momen- 
tum of  moving  bodies  depends  upon  two  things  —  their 
mass  (quantity  of  matter)  and  their  speed.  Then,  in 
general,  we  may  say  that  the  greater  the  mass  of  a  body 
or  the  faster  it  moves,  the  greater  is  its  momentum. 
The  rule  is  commonly  stated  as  follows :  The  momentum 
of  a  body  is  equal  to  the  product  of  its  mass  multiplied  by' 
its  velocity  (speed). 

Examples  of  this  law  are  common.  A  heavy  object 
is  not  so  easily  stopped  as  a  light  one  moving  at  the 
same  rate.  The  faster  a  train  is  moving,  the  more  force 
is  exerted  by  an  obstacle  which  stops  it,  and  the  more 
damage  is  done.  The  faster  you  move  in  riding  a  wheel, 
the  farther  you  can  "  coast "  on  a  level  road.  In  throw- 
ing a  ball,  the  boy  who  can  start  it  at  the  greatest 
speed  throws  it  farthest.  To  test  our  skill  in  throwing 
stones  we  carefully  select  one  of  proper  weight,  some 
being  so  heavy  that  we  cannot  start  them  with  much 
speed,  while  some  are  so  light  that  the  greatest  speed  we 
can  give  them  will  not  make  up  for  their  lack  of  mass. 

61.  Center  of  Gravity.  —  A  body  acted  upon  by  force, 
so  as  to  move  in  a  straight  line,  may  turn  over  and 
over  in  its  flight  (as  a  thrown  pebble  does);  but  one 
point  within  the  body  moves  on  in  a  straight  line,  as  if 


52  MOTION  AND  FORCE 

the  force  had  been  applied  to  that  point  alone.  This 
point  is  the  center  of  mass  of  the  body ;  it  is  the  point 
about  which  the  matter  of  the  body  seems  to  be  evenly 
t  distributed.  If  now  the 

force  of  gravity  acts  upon 
a  body,  whether  it  be  sup- 
ported or  whether  it  be  falling  freely,  the  body  behaves 
as  if  the  force  were  applied  at  its  center  of  mass.  The 
point  may  then  be  called  the  center  of  gravity  (e.g.)  of 
the  body.  We  may  say  that  it  is  the  point  in  a  body  at 
which  the  force  of  gravity  seems  to  be  applied. 

Experiment  45.  —  Try  to  balance  a  ruler  on  your  finger 
(Fig.  32).  Where  is  the  center  of  mass  of  the  ruler  ?  Try  to  bal- 
ance it  upon  a  pencil  point;  mark  the  point.  Is  this  the  center 
of  mass  ?  If  not,  where  is  it  ?  Compare  the  quantity  of  matter  on 
both  sides  of  this  point.  How  do  you  think  the  action  of  gravity 
upon  one  side  of  this  spot  compares  with  that  upon  the  other? 
Where  is  the  e.g.  of  the 

ruler  ?   Now  hang  unequal    J>~  SJ~ 

weights  on  the  ruler,  as  in    A 
Fig.  33.    Try  to  find   the  Fl(J  33 

e.g.  of  the  whole.    Where 

is  it  ?  Compare  the  matter  upon  both  sides  of  the  point.  Where 
is  the  center  of  mass? 

62.  Position  of  the  Center  of  Gravity.  —  A  body  acted 
upon  by  gravity  behaves  as  if  the  force  were  applied  at 
its  e.g.  alone.  If  gravity  really  did  act  only  upon  the 
e.g.,  that  point  would,  of  course,  move  toward  the  e.g. 
of  the  earth  until  stopped  by  some  other  force.  And 
we  find  it  to  be  true  that  any  body  on  earth  that  is  free 
to  move  takes  such  a  position  that  its  center  of  gravity 
shall  be  as  low  as  possible. 


SOME  EFFECTS  OF  NEWTON'S  LAWS 


53 


Experiment  46.  —  Try  to  balance  an  egg  on  its  end.  Explain 
the  result  (Fig.  34).  Do  the  same  with  a  weighted  ball  or  disk 
(Fig.  35).  Hang  a  ball  by  a  thread,  as  in  Fig.  36,  and  move  it 


Fm.  34 


FIG.  35 


to  a  position  a.  Now  where  is  the  e.g.  of  the  whole  pendulum? 
Release  the  ball  and  note  its  behavior.  When  it  comes  to  rest, 
where  is  its  center  of  gravity? 

63.  The  Problem  of  Support.  —  When  a  body  is  fall- 
ing freely  its  e.g.  moves  in  a  straight  line 
towards  the  center  of  the  earth,  nearly.        \ 
This  straight  line  is  called  the  line  of 
direction.    Since   gravity  acts   as   if  the  \ 

force  were  applied  along  this  line,  a  body  \ 

will  not  fall  so  long  as  the  straight  line  from 
its  e.g.  to  that  of  the  earth  passes  through 
its  base,  or  point  of  support. 


c 


O 


Experiment  47.  —  Find  the  e.g.  of  your  ruler 
by  balancing,  and  mark  the  point.    Now  place    ((T) 
the  ruler  on  a  table,  push  it  over  the  edge  little  FIG.  36 

by  little,  and  note  the  position  of  its  e.g.  just  before  it  falls. 

The  base  of  a  body  is  the  area  inclosed  by  straight 
lines  drawn  from  one  to  another  of  its  outer  points  of 


54  MOTION  AND  FORCE 

support   taken   in   order.    Fig.  37   shows    eight  points 
of  support;  the   base  is  the   area  bounded  by  dotted 


FIG.  37  FIG.  38 

lines.    In  Fig.  38  dotted  lines  show  the  base  of  a  person 
standing. 

A  pencil  supported  as  at  c  (Fig.  39)  would  be  said  to 
be  in  a  state  of  equilibrium;  that  is,  the  force  of  gravity 
acting  on  ac  is  just  balanced  by  that  acting  on  be.  If 
Q  b  c  were  moved  a  little, 

GL^=^^^aa^miSllxailsa^ailliiliis_m-^^—^\  ^^— 

these  forces  would  no 
FlG-39  longer  balance,   and 

the  pencil  would  fall  in  the  direction  of  the  greater 
force. 

64.  Stability.  —  A  body  which  is  less  easily  tipped 
over  than  another  is  said  to  be  more  stable.  In  general, 
the  lower  the  center  of  gravity  or  the  broader  its  base,  the 
more  stable  a  body  will  be. 

Experiment  48.  —  Stand  your  pencil  on  its  end  ;  then  lay  it  on 
its  side.  In  which  position  has  it  the  broader  base  ?  In  which  is 
it  the  more  stable  ? 

Experiment  49.  —  Pile  up  three  books  and  test  the  stability  of 
the  pile.  Then  add  as  many  more  as  you  can,  and  test  that. 
Which  pile  is  the  more  stable?  Why? 

Try  to  balance  your  ruler,  first  on  its  side  and  then  on  its  end. 
Which  is  easier,  and  why  ? 


SOME  EFFECTS  OF  NEWTON'S  LAWS 


55 


In  loading  carts  or  in  building  different  structures 
the  heavier  material  is  placed  near  the  bottom,  so  as  to 
make  the  e.g.  as  low  as  possible.  Racing  vessels  balance 
their  enormous 
spread  of  sails 
by  a  heavy  mass 
of  lead  on  the 
keel,  which  car- 
ries the  e.g.  far 
down  (Fig.  40). 


FIG.  40 


65.  Centrifu- 
gal Force.  - 

Since  moving  bodies  tend  to  go  in  straight  lines  (§  55), 
it  is  clear  that  whenever  a  body  moves  in  a  curved  path 
force  must  be  constantly  applied  to  pull  it  out  of  a 
straight  line.  Such  a  force  is  called  centripetal  because 

c  ^~ s,.  it  acts  toward  the  center  of  the 

x  \ 

\  curve.    But   since   every  ac- 

\  tion  has  its  reaction,  centrip- 

\  etal  force  will  be  opposed  by 

*h  |  another  force  tending  to  pull 

/  the  body  away  from  the  center; 

/  this  is  called  centrifugal  force. 


FIG.  41 


Experiment  50.  —  Tie  a  string  to 
a  ball  and  swing  it  rapidly  about 
the  hand  in  a  circle  (Fig.  41).  Do 
you  have  to  use  force  to  hold  it  ?  Why  ?  Suddenly  let  the  ball 
go  free,  and  note  its  motion.  What  direction  does  it  tend  to  take  ? 
Try  the  same  thing  with  a  very  short  string  and  a  very  long  one. 
Explain  any  difference.  Note  that  the  two  forces  exactly  balance 
each  other ;  for  while  one  acts  toward  and  the  other  away  from 


56 


MOTION  AND  FORCE 


the  center  A,  the  ball  moves  no  nearer  to  and  no  farther  from  h  than 
the  length  of  the  string  allows.  As  soon  as  you  let  go,  both  forces 
cease  to  act  and  the  ball  obeys  the  first  law  of  motion  (§  55). 

Effects  of  centrifugal  force  are  common.  A  pail  of 
water  may  be  whirled  in  a  circle  overhead,  centrifugal 
force  holding  the  water  against  the  bottom  of  the  pail 
so  that  none  is  spilled.  The  same  force 
may  cause  a  carriage  or  car  to  tip  over  in 
rounding  a  sharp  curve.  The  wheels  are 
held  in  place  by  the  track  or  road,  while 
the  e.g.,  tending  to  go  on  in  a  straight  line 
(Fig.  42),  passes  outside  the  base.  In  all 
cases,  note  that  the  force  is  greater  if  the 
body  moves  rapidly  or  the  curve  is  sharp. 
Water  would  spill  from  the  pail  which 
was  swung  slowly,  and  freight  trains  take 
curves  much  more  easily  than  expresses. 


FIG.  42 


66.  Falling  Bodies.  —  Bodies  fall  be- 
cause gravity  pulls  them.  Now  since  the 
attraction  of  gravity  depends  upon  the  amount  of  matter 
contained  in  any  body,  it  follows  that  the  greater  the 
mass,  the  more  strongly  gravity  will  pull  it ;  that  is,  a 
heavy  body  will  be  acted  upon  greatly  and  a  lighter 
one  less  strongly.  The  result  is  that  all  bodies  will  fall 
equal  distances  in  equal  periods  of  time,  when  not  hindered 
by  any  other  force.  Most  bodies  do  fall  equally  fast ; 
but  a  few  (such  as  feathers,  leaves,  and  paper)  have  so 
large  a  surface,  compared  with  their  weight,  that  their 
falling  is  greatly  hindered.  In  a  vacuum  a  penny  and 
a  feather  would  fall  exactly  together  (Fig.  43). 


SOME  EFFECTS  OF  NEWTON'S  LAWS  57 

Experiment  51.  —  Drop  pieces  of  different  substances  (wood, 
stone,  iron,  lead,  and  others)  from  the  same  height  exactly 
together,  and  note  whether  or  not  they  strike  together. 
Repeat  several  times,  for  accuracy. 

Compare  with  these  the  fall  of  a  leaf  or  sheet  of 
paper.    Note  and  explain  any  differences. 

Falling  bodies  offer  almost  the  only  common 
example  of  motion  which  is  not  opposed  by 
any  considerable  force ;  for  generally  only  the 
air  hinders  their  progress,  and  its  force  is  not 
great.  Thus  it  is  interesting  to  note  this  sort 
of  motion  carefully.  It  has  been  found  that 
a  body  will  fall  about  sixteen  feet  in  one  second. 
But  at  the  end  of  that  second  its  momentum 
alone  is  great  enough  to  carry  it  about  thirty- 
two  feet  in  a  second.  The  result  is  that  in 
the  second  second  the  body  will  travel  thirty- 
two  feet  because  of  its  momentum  (or  inertia) 
and  sixteen  feet  by  force  of  gravity,  making  a 
total  of  forty-eight  feet.  So  as  it  goes  on  it 
loses  little  or  none  of  its  momentum  and  con- 
stantly gathers  more,  as  gravity  keeps  acting  upon  it ; 
so  that  the  farther  a  body  falls,  the  faster  it  goes.  This 
is  why  a  long  fall  generally  does  more  damage  than  a 
short  one. 

67.  Pendulum.  —  A  pendulum  is  a  device  so  sup- 
ported that  it  is  free  to  swing  to  and  fro  about  a  fixed 
point.  Fig.  44  shows  a  pendulum,  a  being  its  point  of 
support  (on  which  it  swings)  and  b  the  weight  or  bob. 
Lift  b  to  the  position  c  and  let  it  go ;  gravity  acts  upon 
it,  pulling  the  bob  downward  toward  e.  At  the  position 


58 


MOTION  AND  FORCE 


e  gravity  ceases  to  pull  b  downward ;  but  the  bob  then 
has  enough  momentum  so  that  it  rises  to  d  against  the 
opposing  force  of  gravity.  At  d  the  bob  stops,  gravity 
now  pulls  it  to  e,  and  it  then  moves  on  toward  c.  The 
path  in  which  b  swings  (ced)  is  called  the  arc  of  the 
pendulum.  A  single  complete  sweep  across  this  arc  is 
called  one  vibration.  As  a  pendulum  swings  to  and 

fro,  its  arc  constantly  be- 
comes smaller,  and  in  time 
the  bob  comes  to  rest  at  e. 
The  air  offers  a  slight 
resistance  to  the  moving 
body,  slowly  bringing  it 
to  rest. 

Experiment  52.  —  Make  two 
pendulums  of  exactly  equal 
lengths,  by  tying  string  to 
stones.  Make  them  about  two 
feet  long,  using  stones  of  very 

unequal  weights.  Start  them  exactly  together  and  compare  the 
rates  of  their  vibrations,  that  is,  the  number  of  swings  made  by 
each  in  a  certain  period  of  time.  What  effect  has  the  weight  of 
the  bob  upon  the  vibration  rate  of  the  pendulums  ? 

Experiment  53.  — Swing  a  pendulum  through  a  small  arc  and 
count  its  vibrations  for  15  seconds.  Now  swing  the  same  pendu- 
lum through  an  arc  much  greater,  and  count  its  vibrations  for  15 
seconds.  What  effect  has  the  length  of  arc  upon  the  rate  of 
vibration  ?  (The  length  of  the  arc  makes  a  slight  difference  in 
rate  if  one  arc  is  much  greater  than  the  other,  and  none  at  all  if 
both  arcs  are  small  —  less  than  3°.) 

Experiment  54 .  —  Make  a  pendulum  9  inches  long  and  another 
36  inches  long.  Carefully  count  the  vibrations  of  each  for  15 
seconds  and  compare  results.  Now  make  one  4  inches  long  and 


SOME  EFFECTS  OF  NEWTON'S  LAWS  59 

another  16  inches  long  and  compare  their  rates  of  vibration. 
How  much  longer  is  the  second  than  the  first  ?  Which  vibrates 
the  faster  ?  How  much  faster  ? 

What  one  thing  do  you  find  to  make  a  marked  difference  in 
the  vibration  rate  of  a  pendulum  ?  Try  to  make  a  statement  of 
the  effect  of  length  upon  the  rate  of  vibration. 

QUESTIONS 

1.  What  is  inertia?    State  examples.    Why  can  you  not  start 
a  bicycle  at  once  at  your  greatest  speed  ? 

2.  What  is  momentum?    Upon  what  two  factors  does  the 
momentum  of  a  body  depend?    How  is  it  generally  measured? 

3.  A  rifle  ball  weighing  half  an  ounce  moves  at  the  rate  of  one 
thousand  feet  a  second,  while  a  forty-pound  cannon  ball  moves  at 
the  rate  of  one  foot  per  second.  Which  has  the  greater  momen- 
tum ?    By  which  would  you  rather  be  struck  ?    Why  ? 

4.  Why  does  a  woodcutter  sometimes  fasten  his  ax  in  a  stick 
and  then  invert  it,  striking  the  block  with  the  stick  uppermost  ? 

5.  Why  can  you  not  stand  an  egg  on  its  end?    If  there  were 
a  hole  straight  through  the  earth's  center  from  surface  to  surface, 
how  far  into  it  would  a  falling  body  go  ? 

6.  Under  what  conditions  will  a  body  be  supported  from 
falling? 

7.  Upon  what  does  the  stability  of  a  body  depend,  and  how  ? 
Why  is  it  hard  to  walk  upon  stilts  ?    Why  spread  your  feet  apart 
to  receive  a  blow  in  boxing  ? 

8.  Explain  the  cause  of  centrifugal  force.    State  examples  of  it. 
Why  do  you  lean  in  turning  a  corner  ?    Why  is  the  inside  rail  of 
a  track  placed  lower  ?  What  conditions  increase  centrifugal  force  ? 

9.  How  far  will  a  body  fall  in  one  second?    in  two  seconds? 
Why  does  a  body  constantly  increase  in  its  speed  as  it  falls? 
Why  is  more  damage  done  by  a  longer  fall,  as  a  rule? 

10.  Describe  a  pendulum.  What  force  causes  it  to  swing  down- 
ward ?  Why  does  it  then  swing  upward  ?  If  no  force  but  gravity 
opposed  its  upward  swing,  how  far  would  it  go  as  compared  with 
its  downward  swing  ? 


60  MOTION  AND  FORCE 

11.  Which  has  the  faster  vibration  rate,  a  short  or  a  long 
pendulum?    If  a  clock  loses  time,  would  you  make  its  pendulum 
longer  or  shorter  in  regulating  it  ? 

12.  Since  a  pendulum  is  made  to  vibrate  by  the  force  of  grav- 
ity, would  it  swing  faster  or  slower  on  a  mountain  top  than  in  a 
valley?    (See  §  29.) 

SECTION  III 
WORK  AND  MACHINES 

68.  Work.  —  Work  is  said  to  be  done  whenever  a  force 
causes  motion.    From  this  it  is  clear  that  work  may  be 
measured  in  terms  of  the  amount  of  motion  caused  by  a 
certain  force.    The  amount  of  work  done*  by  a  force  is 
commonly  expressed  in  foot  pounds.     One  foot  pound 
is  the  amount  of  work  done  in  raising  one  pound  of 
matter  a  distance  of  one  foot  against  gravity. 

The  rate  at  which  work  may  be  done  is  sometimes 
called  power.  The  ability  of  an  engine,  for  example,  to 
do  work  is  expressed  as  so  many  "horse  power.  One  horse 
power  is  the  ability  to  do  thirty-three  thousand  foot  pounds 
of  work  a  minute. 

69.  Machines.  —  A  machine  is  a  device  which  helps 
man  to  do  work.    Note  that  a  machine  cannot  of  itself 
do  work ;    it  cannot  make  energy.    It  can  only  help  in 
applying  force  to  good  advantage ;  and  as  every  machine 
uses  up  some  of  the  energy  in  its  own  motion,  none  gives 
us  quite  as  much  work  as  is  done  upon  it. 

70.  Uses  of  Machines. — In  spite  of  this  fact,  however, 
there  are  several  things  gained  by  the  use  of  machines, 
which  more  than  make  up  for  this  loss  in  work. 


WORK  AND  MACHINES 


61 


1.  They  help  man  to  apply  force  in  a  more  convenient 
direction.     The  pulley  (Fig.  45)  and  lever  (Fig.  46)  are 
common  examples  of  this.    A  bit  of 

thought  will  show  how  handy  it 
may  be,  at  times,  to  thus  change 
the  direction  of  motion. 

2.  We  may  use   other  forces 
than  our  own  to  run  them.    Steam 
engines,  windmills,  electric  mo- 
tors,  water  wheels,   and  tread- 
mills all  serve  to  call  such  forces 
to  mind. 

3.  They  help  us  to  store  energy 
to  be  used  at  another  time.    For 
example,  the  spring  of  a  watch, 
in  unwinding,  does  only  the  work 
which  was  done  upon  it  in 
winding  it  up.  We  could  not 


FIG.  45 


easily  exert  force  directly  upon  the  wheels  all  day  long. 

4.  By  their  use  we  may  exchange  strength  of  force  for 

speed,  and  speed  for  strength  of  force.    That  is,  we  may 


62 


MOTION  AND  FORCE 


use  great  force  slowly  and  cause  a  small  body  to  move 
rapidly,  or  use  small  force  rapidly  and  move  a  great 
I  1      weight  slowly.    A  few  ex- 

S  amples  of  this  will  be  given 


Weight 


Force 


FIG.  46 


in  §§  72  and  73. 


71.  Law  of  Machines.  —  The  fourth  of  these  uses  is 
one  of  much  importance.    It  will  be  more  easily  under- 
stood when  we  have  clearly  in  mind  the  following  Law 
of  Machines:   The  force  and  the  resistance  vary  inversely 
as  the  distances  through  which  each   acts.    This   means 
that  if  a  certain  force  causes  motion  against  a  resist- 
ance that  is  greater  or  smaller  than  itself,  the  distance 
through  which  the  resistance  acts  must  be  just  as  many 
times  smaller  or  greater  than 

the  distance  through  which  the 
force  is  applied. 

72.  Pulleys   and  Levers.  - 
A    few  simple   machines  will 
serve  as  examples  of  this  law. 
Fig.  47  shows  one  movable  pul- 
ley B  attached  to  a  weight  W\ 
F  is  the   point  at  which  the 
force  is  applied.    Notice  that 
as  F  moves,  W  will  move  only 
half  as  far.    From  the  law,  this 
shows  that  the  force  need  be 
only  half  as  great  as  the  weight. 

With  two  movable  pulleys  the  force  would  act  through 
four  times  the  distance  and  lift  a  weight  four  times 
as  heavy. 


FIG.  47 


WORK   AND  MACHINES 


63 


A  lever  is  any  device  having  force  applied  at  one 
point  and  resistance  at  another,  the  whole  turning  on  a 
point  called  a  fulcrum.  A  crowbar  may  be  used  as 
a  lever  (Fig.  48).  a, 

As  the  force  acts  ^^\ 

from  a1  to  6',  the  \ 

weight  moves  only 

]&' 


FIG.  48 


from  a  to  b ;  hence 
a  small  force  at 
a1  will,  in  acting 
through  a  greater  distance,  a'b',  move  a  greater  weight 
at  a  the  lesser  distance,  ah.  Also  a  great  force  at  b  could 
move  a  lesser  weight  at  b'  with  greater  speed.  Fig.  49 

shows  three  classes  of 
levers.  In  Class  I  the 
fulcrum/  is  between  the 
points  where  the  force  p 
and  the  resistance  w  are 
applied.  In  Class  II /  is 
at  the  end,  w  being  ap- 
plied between  it  and  p  ; 
this  lever  gives  us  a 
gain  in  force  at  the 
expense  of  speed.  In 
Class  III  the  force  p  is 
applied  beween  the  ful- 
crum and  the  resistance,  so  that  we  gain  in  speed  at 
the  expense  of  force. 

Experiment  55.  —  Pulleys  and  levers  are  common,  and  many 
experiments  may  be  made,  according  to  the  time  and  mate- 
rial available.  A  movable  pulley  is  not  hard  to  find ;  but  for  a 


fl 


P 


w 


III 
FIG.  49 


64  MOTION  AND  FORCE 

substitute  smooth  steel  screw  eyes  may  be  used  with  small  hard 
thread. 

Examples  of  levers  are  always  at  hand.  In  the  following,  name 
the  class  to  which  each  belongs  and  state  whether  we  gain  in 
force  or  in  speed  by  using  it :  scissors  ;  a  common  pump  handle  ; 


FIG.  51 

pincers;  sugar  tongs;  steelyards;  nutcrackers;  a  crowbar  when 
its  fulcrum  is  on  the  ground  beneath  a  weight ;  ice  tongs ;  a  tin- 
smith's shears  ;  a  wheelbarrow ;  a  claw  hammer  (Fig.  50). 

73.  Other  Simple  Machines.  —  The  screw  is  generally 
used  to  gain  intensity  of  force.  As  the  force  is  applied 
to  the  circumference  of  a  wheel  b  (Fig.  51),  the  sur- 
face on  which  the  resistance  acts  will  move  ahead  only 
the  small  width  of  one  thread  of  the  screw.  Since  the 
force  is  applied  through  a  far  greater  space  than  the 
resistance,  the  gain  in  force  is  great. 

Fig.  52  shows  a  wheel  and  axle.  The  axle  is  much 
smaller  than  the  wheel  and  turns  with  it.  A  small 
force  F  applied  on  the  wheel  may  move  a  much  greater 
weight  E  on  the  axle;  but  E  moves  proportionally 
slower  than  F,  —  that  is,  a  great  force  at  E  will  move 


WORK  AND  MACHINES 


65 


a  small  weight  at  F  with  a  gain  in  speed.    In  a  windlass 
and  a  capstan  this  device  is  used  to  gain  in  force. 

G-ear  wheels  (Fig.  53)  are  used  in  a  similar  way.    If  a 
large  wheel  runs  in  a  smaller,  the  gain  is  in  speed ;  but 


FIG.  52 


FIG.  53 


if  force  is  applied  to  the  smaller  wheel,  the  larger  turns 
more  slowly  but  exerts  greater  force.  Gear  wheels  are 
commonly  used  in  machinery. 

The  inclined  plane  is  used  for  a  gain  of  force  at  a  loss 
of  speed.  A  plank  inclined  from  the 
ground  to  a  wagon  floor  enables  a  man 
to  get  a  heavy  body  into  his  cart.  The 
more  gradual  the  slant,  the  more  he  gains 
in  force  required.  A  wedge  (Fig.  54) 
has  two  inclined  faces.  It  also  gains  for 
us  intensity  of  force  at  the  expense  of 
speed. 

FIG.  54 

Experiment  56.  — A  vise,  copy  press,  thumb- 
screw, or  bolt  and  wrench  may  serve  to  experiment  with  the 
screw.    For  a  wheel  and  axle,  any  grooved  wheel  with  its  axle 


66  MOTION  AND  FORCE 

fixed  so  as  to  turn  with  it  may  serve,  or  one  can  easily  be  made. 
An  old  clock  will  furnish  gear  wheels.  An  inclined  plane  can 
be  made  wherever  convenient,  and  a  thick  knife  blade  will  do 
for  a  wedge. 

QUESTIONS 

1.  Define  work.    How  is  work  measured  ?    What  is  the  unit 
of  work  ? 

2.  What  is  meant  by  power  ?    What  is  the  unit  of  the  rate  of 
doing  work  ?    How  much  is  one  foot  pound  ?  one  horse  power  ? 

3.  What  is  a  machine?    Can  a  machine  do  work  of  itself? 
Does  a  machine  gain  or  lose  work  ? 

4.  What,  in  general,  is  the  use  of  machines  to  man  ?    Name 
four  special  uses  of  them.    Illustrate  each. 

5.  State  the  Law  of  Machines.    Show  how  a  lever  applies  this 
law.    Is  a  movable  pulley  used  to  gain  force  or  speed  ? 

6.  Why  do  tailors'  shears  have  long  blades  and  short  handles, 
while  plumbers'  shears  have  short  blades  and  long  handles  ? 

7.  Why  does  a  bicycle  of  high  gear  run  harder  than  one  of 
low  gear? 

8.  What  is  the  advantage  gained  in  using:  a  single  pulley? 
a  windmill?  a  coiled  spring  in  a  watch?  the  walking  beam  on  an 
engine?  a  wheel  and  axle? 

9.  State  the  advantage  given  by  a  second-class  lever  and  by 
a  third-class  lever.    What  can  you  say  of  the  advantage  in  levers 
of  the  first  class  ? 

10.  Name  as  many  examples  of  levers  in  use  as  you  can. 
Name  some  familiar  uses  of  the  screw  and  the  wedge. 

11.  Explain  the  use  of  gear  wheels  in  machinery. 


CHAPTER   IV, 

HEAT  AND   ENERGY 

SECTION   I 

HEAT 

74.  Sources  of  Heat.  —  The  sun  is  a  most  important 
source  of  heat  on  earth,  for  without  its  rays  the  atmos- 
phere would  be  intensely  cold  and  we  should  not  have 
the  supplies  of  wood,  coal,  and  oil  which  are  used 
as  fuels.  Other  sources  of  heat  are  illustrated  in  the 
following  experiments. 

Experiment  57. —  Using  a  convex  lens,  focus  the  sun's  rays 
upon  a  piece  of  tissue  paper  for  a  moment  (§139).  Note  their 
effect  on  the  paper.  Name  other  examples  of  the  heating  effect 
of  the  sun's  rays. 

Experiment  58.  —  Friction.  File  a  soft  iron  nail  for  a  moment 
and  then  feel  of  the  filed  surface.  Saw  through  a  piece  of  wood 
and  feel  of  the  saw.  Rub  a  metal  button  on  a  smooth  piece  of 
cloth.  Name  any  examples  of  bodies  heated  by  friction.  The 
bearings  of  car  wheels  often  become  very  hot.  Why  ? 

Experiment  59.  —  Percussion.  Hammer  a  small  piece  of  lead  for 
half  a  minute  and  feel  of  it.  Repeat  this,  using  a  soft  iron  nail.  Did 
you  ever  pick  up  a  rifle  bullet  that  had  just  been  flattened  by  strik- 
ing an  iron  target  ?  Think  of  other  cases  of  heating  by  percussion. 

Experiment  60.  —  Compression.  Pump  air  into  a  bicycle  tire 
for  a  few  moments  and  then  feel  of  the  pump.  Can  you  discover 
evidence  of  heat  being  developed  by  compression  ? 

Experiment  61.  —  Chemical  Action.  Pour  a  little  hydrochloric 
acid  upon  bits  of  zinc  in  a  test  tube.  Very  carefully  and  slowly 

67 


68  HEAT  AND  ENERGY 

pour  a  little  sulphuric  acid  upon  water  in  a  test  tube.  In  each 
case  feel  of  the  glass  around  the  liquid.  What  do  you  discover 
about  chemical  action? 

Heat  is  very  coAimonly  caused  by  combustion  or  burn- 
ing. This  is  a  sort  of  chemical  action  and  is  treated  in 
§260.  Electricity  is  also  a  common  source  of  heat; 
its  heating  effects  are  shown  in  electric  lights  and  are 
used  in  furnaces  and  heaters.  Its  action  is  explained 
in  §  192. 

75.  Theory  of  Heat.  —  In  studying  the  molecular 
theory  (§  11)  we  learned  that  the  molecules  of  all  matter 
are  thought  to  be  in  a  state  of  constant  vibrating  motion. 
Naturally  we  may  suppose  that  in  some  bodies  the  vibra- 
tion is  more  rapid  than  in  others ;  also  that  in  the  same 
body  the  motion  may  be  greater  or  less  at  different 
times.  The  heat  of  any  body  is  believed  to  vary  with 
this  vibration  of  its  molecules,  as  stated  in  the  theory 
of  heat  as  follows :  The  heat  of  a  body  is  the  energy  of 
vibration  of  its  molecules  ;  the  faster  they  move,  the  warmer 
is  the  body. 

With  this  theory  in  mind,  the  results  obtained  in 
Experiments  58  and  59  may  be  easily  understood. 
Rubbing,  in  the  one  case,  and  pounding,  in  the  other, 
simply  caused  the  motion  of  the  molecules  to  become 
more  rapid,  and  the  masses  became  warmer.  The  theory 
applies  also  in  the  other  cases. 

Within  certain  limits  we  can  discover  differences  in 
the  heat  of  things  about  us ;  we  say  that  a  body  feels 
more  or  less  "  warm."  It  must  be  carefully  noted,  how- 
ever, that  this  is  only  the  effect  that  heat  produces  upon 


HEAT  69 

our  sense.  We  must  not  judge  of  the  nature  of  heat  by 
this  single  effect,  for  it  is  only  one  of  many  different 
effects.  It  is  important,  in  order  to  understand  the 
further  study  of  this  chapter,  that  we  fix  firmly  in 
mind  the  idea  that  heat  is  a  form  of  energy  —  the  energy 
of  molecular  motion. 

76.  Cold.  —  Cold  means  simply  the   absence  of  heat. 
Since  heat  is  molecular  energy,  and  the  molecules  of 
every  mass  are  in  motion,  it  follows  that  no  body  has 
absolutely  no  heat.    Thus  complete  cold  is  unknown. 
We  use  the  word  cold  to  express  a  condition  of  less  heat 
than  some  other  substance  has. 

77.  Temperature.  —  Temperature  is  the  condition  of 
a  body  with  regard  to  the  intensity  of  its  heat.    If  a 
body  is  warmer  than  another,  we  say  that  it  has  a  higher 
temperature;  if  colder,  we  say  its  temperature  is  lower. 

Care  must  be  taken  to  avoid  calling  temperature  the 
"  quantity  of  heat "  of  a  body.  A  cupful  of  water  might 
have  a  higher  temperature  than  water  in  a  kettle ;  but 
at  the  same  time  the  kettleful  would  have  a  greater 
quantity  of  heat,  because  there  is  so  much  more  water. 
Temperature  is  the  average  heat  of  each  particle,  while 
quantity  of  heat  is  the  average  of  each  particle  multi- 
plied by  the  number  of  particles. 

78.  The  Thermometer.  —  The  thermometer  is  a  device 
for  measuring  temperature.    It  does  this  by  expressing,  in 
degrees,  how  much  warmer  or  colder  a  body  is  than  some 
other  substance  taken  as  a  standard.    Two  thermometers 
are  in  common  use,  the  Centigrade  and  the  Fahrenheit. 


70 


HEAT  AND  ENERGY 


The  only  difference  is  in  the  marking  of  the  scale  of 
degrees,  as  shown  by  the  two  side  by  side  in  Fig.  55. 
The  Centigrade  scale  is  largely  used  in  scientific  work. 
Its  standard  is  freezing  water  and  is  marked  zero  (0). 
The  temperature  of  boiling  water  is  marked 
one  hundred  (100).    The  space  between  these 
marks   on   the    scale    is    divided   into    100 
equal  parts,  each  called  a  degree  (°). 

The  Fahrenheit  scale  is  most  commonly 
used  by  us.  Its  zero  is  the  temperature  of 
a  mixture  of  ice  and  salt,  and  the  boiling 
point  of  water  is  212°.  Water  freezes  at 
32°  above  zero  on  this  scale. 

Experiment  62.  —  Carefully  test  these  substances 
(freezing  water,  boiling  water,  and  ice  and  salt) 
with  both  thermometers.  Compare  the  tempera- 
tures of  several  substances  on  both  scales,  and  try 
to  discover  a  rule  for  changing  one  reading  to  the 
same  temperature  on  the  other  scale. 

79.  How   a   Thermometer  is  made.  —  A 

small  tube  of  hairlike  bore,  having  a  bulb  at 
one  end,  is  partly  filled  with  mercury.    The 
air  is  removed,  because  its  pressure  would 
prevent  the  mercury  from  rising,  and  the  tube 
is  completely  closed.     Mercury  will  expand 
when  heated  (see  §  81)  and  shrink  when 
cooled,  so  that  as  the  temperature  rises  or 
falls,  the  mercury  moves  up  or  down  the  fine 
FIG.  55         tube.    Assuming  that  its  expansion  is  uni- 
form, we  may  compare  temperature  changes  by  compar- 
ing the  distances  that  the  mercury  column  rises  or  falls. 


EFFECTS  OF  HEAT  71 

To  mark  the  scale,  the  bulb  is  put  into  ice,  and  the  point  to 
which  the  mercury  rises  is  marked  0 ;  the  bulb  is  then  put  into 
steam  or  boiling  water,  and  the  point  to  which  the  mercury  rises  is 
marked  100.  The  space  between  is  then  divided  into  equal  parts, 
and  the  marks  may  be  continued  above  100  and  below  0.  This 
gives  a  Centigrade  scale.  How  would  the  marking  of  a  Fahrenheit 
scale  differ  from  this? 

QUESTIONS 

1.  State  the  theory  of  heat.    Give  examples  which  seem  to 
show  the  truth  of  this. 

2.  What  is  the  great  source  of  heat  upon  earth?    Can  you 
show  how  the  heat  from  coal  once  came  from  the  sun  ? 

3.  What  is  meant  by  cold?    Is  any  body  absolutely  cold?    If 
a  body  were  entirely  cold,  what  would  be  the  condition  of  its 
molecules  ? 

4.  Define  temperature.  Carefully  explain  the  difference  between 
temperature  and  quantity  of  heat. 

5.  For  what  is  a  thermometer  used?    Explain  how  the  ther- 
mometer is  made  and  how  it  acts. 

6.  What  two  thermometers  are  in  common  use  ?    Which  one 
do  we  use  daily?    What  is  the  standard  in  each?    On  which 
scale  are  the  degrees  the  shorter? 

7.  Name  and  describe  the  more  common  sources  of  heat. 


SECTION  II 
EFFECTS   OF  HEAT 

80.  The  Effects  named.  —  In  general,  the  effects  of 
applying  heat  to  bodies  are  four  in  number,  —  chemical 
effects,  electrical  effects,  changes  of  volume,  arid  changes 
of  state.  The  first  two  of  these  effects  will  be  treated  in 
later  chapters ;  we  shall  now  consider  only  changes  of  vol- 
ume and  of  state  which  are  caused  by  the  action  of  heat. 


72  HEAT  AND  ENERGY 

81.  Changes  of  Volume. —  When,  without  more  matter 
being  added,  a  body  grows  larger,  it  is  said  to  expand  ; 
when,  without  losing  any  of  its  particles,  a  body  grows 
smaller,  it  is  said  to  contract.    As  a  general  rule,  masses 
expand  when  they  are  heated  arid  contract  when  cooled. 

Experiment  63.  —  Secure  a  hollow  metal  ball  which  exactly  fits 
into  a  ring  (Fig.  56).  Heat  the  ball,  and  see  if  it  can  be  forced 
through  the  ring.  Heat  both  ring  and  ball  and  try  them  again ; 
they  should  fit.  Cool  the  ball  and  heat 
the  ring.  How  do  they  fit  now? 

Carefully  measure  a  long  iron  nail. 
Heat  it  thoroughly  and  measure  again. 
Is  it  longer  or  shorter? 

Experiment  64.  —  Fill  a  test  tube  with 
water  and  fit  a  stopper  lightly  into  its 
mouth.  Heat  the  water  and  note  the  re- 
sult. Explain  this.  (Do  not  crowd  the 
stopper  or  heat  the  water  too  highly.) 

Fill  a  long  narrow  tube  with  hot  water ; 
FIG  56  let  it  cool  to  an  ordinary  temperature  and 

note  any  change  in  volume. 

Experiment  65 Arrange  a  tube  so  as  to  run  through  the 

stopper  of  a  flask  or  bottle,  into  a  vessel  of  water,  as  in  Fig.  57. 
Heat  the  flask  and  explain  what  you  observe.  What  is  in  the 
flask?  What  change  does  it  undergo? 

Now  remove  the  heat,  watching  the  tube  carefully.  As  the 
flask  cools,  what  change  takes  place  in  its  contents?  Try  to 
account  for  what  you  notice. 

82.  Uses  of  Expansion  and  Contraction.  —  From  these 
experiments  we  see  that  liquids  and  gases  may  expand 
and  contract  as  well  as  solids.    The  value  of  this  will 
be    seen    when    we    study    convection   (§  91).    In    the 
case  of  solids  this  principle  is  commonly  used  to  good 


EFFECTS  OF  HEAT 


73 


advantage,  for  the  force  exerted  by  a  body  in  expanding 
or  contracting  is  very  great.  To  make  a  wagon  tire  fit 
tightly,  a  blacksmith  often  puts  it  on  after  heating  it; 
upon  cooling,  it  contracts  and  fits  the  wheel  closely. 
Similarly  the  parts  of  boilers,  bridges,  and  other  steel 


FIG.  57 

structures  are  fastened  with  rivets  which  are  put  in 
while  red-hot;  these  cool  and  contract,  drawing  the 
parts  tightly  together. 

83.  Exceptions  to  the  Rule.  —  A  few  substances  do 
not  obey  the  general  rule  for  expansion  and  contrac- 
tion. Of  these,  water  is  a  common  example.  We  have 
seen  ice  floating  upon  water,  which  shows  that  it  is 
lighter  than  the  liquid ;  but  as  ice  is  only  frozen  water, 
we  know  that  it  must  have  expanded  upon  cooling. 


74  HEAT  AND  ENERGY 

Experiment  66.  —  Fill  two  thin  bottles  with  water  and  cork 
them  tightly.  Heat  one  and  allow  the  other  to  freeze.  The  water 
may  be  left  to  freeze  over  night ;  but  do  not  try  to  heat  the 
tightly  closed  bottle  without  help  from  your  instructor.  Note 
and  draw  conclusions  from  the  results. 

Careful  study  has  shown  that  water  has  its  smallest 
volume  at  4-°  Centigrade.  If  heated  above  or  cooled 
below  that  point,  it  expands. 

84.  Changes  of  State.  —  Early  in  our  study  we  learned, 
that  solids  change  to  liquids  and  liquids  to  gases  upon 
being  heated  ;  also  that  gases  become  liquids  and  liquids 
change  to  solids  when  cooled  (§  9).  In  some  substances 
these  changes  occur  at  ordinary  temperatures,  and  are 
common  enough.  In  other  substances,  however,  the 
changes  would  need  such  a  high  or  low  degree  of  heat 
that  they  are  seldom  or  never  accomplished. 

The  change  from  a  solid  to  a  liquid  is  called  melting, 
fusion,  or  liquefying;  the  change  from  a  liquid  to  a  solid 
is  called  solidifying.  The  temperature  at  which  a  solid 
substance  liquefies  is  the  same  as  that  at  which  it  solidi- 
fies from  a  liquid  state.  Vaporization  is  the  change  from 
a  liquid  to  a  gaseous  state;  the  temperature  at  which  a 
substance  vaporizes  is  called  its  boiling  point. 

In  a  large  number  of  substances  pressure  upon  them 
raises  the  temperature  at  which  these  changes  of  state 
occur.  Thus  water,  which  can  usually  be  no  hotter  than 
100°  C.,  rises  to  a  much  higher  degree  in  a  locomotive 
boiler,  where  it  is  under  pressure  from  the  steam.  With 
substances  such  as  ice,  which  contract  when  they  are 
melting,  pressure  lowers  the  melting  point.  Thus  a 
block  of  ice  receives  the  imprint  of  a  dish  which  rests 


EFFECTS   OF   HEAT  75 

upon  it  heavily,  the  ice  beneath  the  dish  melting  faster 
than  the  other  parts. 

85.  Evaporation We  know  that  a  moist  cloth  soon 

dries  if  hung  in  warm  air;   also  that  a  thin  layer  of 
water  in  a  dish  or  on  some  hard  surface  soon  disappears. 
Clearly  the  liquid  must  have  gone  somewhere.    But  did 
it  pass  off  in  a  liquid  state?    If  so,  we  should  probably 
have  seen  it  go.    We  have   to  suppose,  then,  that  it 
changes  into  a  gas  and  passes  off"  into  the  air,  and  we  say 
that  it  has  evaporated.    Evaporation  may  be  denned  as 
that  sort  of  vaporization  which  goes  on  quietly  at  ordi- 
nary temperatures. 

Note  that  evaporation,  not  being  produced  necessarily 
by  boiling,  depends  partly  upon  the  ability  of  the  atmos- 
phere to  receive  the  vapor.  Of  course  some  substances 
vaporize  more  easily  than  others,  but  in  general  the 
conditions  which  aid  evaporation  are  conditions  in  the 
air  surrounding  the  liquid.  Warm  air  can  hold  more 
vapor  than  cold;  dry  air  can  naturally  take  on  more 
than  that  which  is  moist  or  humid;  and  evaporation 
goes  on  faster  when  the  atmosphere  is  in  motion.  Thus 
the  best  conditions  would  be  warm,  dry,  moving  air. 

Experiment  67.  —  Try  these  different  conditions  with  small 
amounts  of  water.  Also  use  such  liquids  as  alcohol,  ether,  and 
naphtha.  Blow  upon  them,  and  see  if  there  is  any  faster  evapora- 
tion. Why  put  damp  clothes  in  a  warm  place  to  dry?  Will 
clothes  dry  when  frozen  ?  Do  they  dry  better  on  windy  days  ? 

86.  Condensation.  —  The  amount  of  vapor  which  air 
can  hold  varies  with  its  temperature;  other  things  being 
equal,  the  warmer  the  atmosphere,  the  more  vapor  it  can 


76  HEAT  AND  ENERGY 

hold.  When  air  at  any  temperature  holds  all  that  it 
can,  it  is  said  to  be  saturated.  If  now  it  be  somewhat 
cooled,  this  air  can  no  longer  hold  all  the  vapor  that  is 
in  it,  and  some  will  change  back  to  its  liquid  state. 
This  change  is  called  condensation.  The  condensed  water 
vapor  may  then  float  about  as  tiny  liquid  drops;  small 
masses  of  these  drops  may  pass  to  another  place  and 
there  evaporate  again,  like  the  cloud  from  a  locomotive  ; 
while  large  masses  would  form  a  fog  or  cloud.  If  the 
drops  were  large  they  would  fall  as  rain. 

Experiment  68.  —  Put  ice  or  snow  into  a  pitcher  and  take  it 
into  a  warm  room.  Watch  the  outside  of  the  pitcher,  and  explain. 
Breathe  upon  a  cold  piece  of  glass.  Why  does  frost  form  on  the 
inside  of  a  window  pane  ?  Why  do  we  "  see  our  breath  "  in  cold 
weather  ? 

87.  Distillation.  —  Important  use  is  made  of  these 
principles  (§§  84-86)  in  separating  substances  from  each 
other  or  from  impurities.  Since  different  sorts  of  mat- 
ter vaporize  at  different  temperatures,  a  mixture  may  be 
heated  to  the  low  boiling  point  of  one  substance  without 
vaporizing  the  others ;  the  gas  from  this  one  may  then 
be  cooled,  giving  us  the  desired  liquid  or  solid,  free 
from  the  others.  The  process  is  called  distillation. 

Experiment  69.  —  A  device  for  distillation  may  be  arranged  as 
in  Fig.  58.  Instead  of  the  condenser  c,  a  long  tube  of  glass  or 
metal  may  be  run  through  a  trough  in  which  cold  water  is  flowing. 
Fig.  58  shows  the  condenser  as  generally  used  in  distilling. 
Muddy  water  may  be  boiled  in  a  closed  flask  /;  the  steam  runs 
through  a  tube  t  which  carries  it  to  the  coiled  tube  e  in  the  con- 
denser. Cold  water  running  into  c  from  a  pipe  p  surrounds  the 
coiled  tube  e  and  runs  out  at  o.  The  steam  in  e  is  cooled  and 


EFFECTS   OF  HEAT 


77 


FIG.  58 


condensed  by  the  cold  water  around  it,  running  out  at  n  as  water. 
Since  only  steam  passed  from /to  e,  the  water  should  come  from 
the  tube  clear  and 

pure.    It  is  called  , t 

distilled  water. 

Distillation  is 
used  on  ocean 
steamers  to  sup- 
ply water  for 
the  boilers;  salt 
sea  water  would 
rust  them  badly, 
but  distilling 
removes  the 
salt.  The  pro- 
cess is  also  used 
in  making  alcohol  and  liquors,  flavoring  extracts,  per- 
fumes, and  many  other  compounds. 

88.  Latent  Heat. --The  temperature  at  which  ice 
melts  (32°  F.)  is  the  same  as  that  at  which  water 
freezes.  If  a  piece  of  ice  is  put  into  a  vessel  and  slow 
heat  applied,  the  ice  changes  to  water;  but  so  long  as 
any  ice  remains,  the  temperature  of  the  water  remains 
the  same  as  that  of  the  ice.  In  other  words,  though 
much  heat  is  applied  to  the  mass  its  temperature  does 
not  rise.  This  fact  was  early  discovered,  and  the  name 
latent  (i.e.  hidden)  was  given  to  the  heat  which  thus 
seemed  to  disappear. 

The  same  thing  occurs  when  other  substances  are 
changed  from  solids  to  liquids,  or  from  liquids  to  gases. 
It  is  also  true  that  the  heat  thus  taken  into  a  body  is 


78  HEAT  AND  ENERGY 

given  out  again  whenever  the  mass  changes  back  to 
its  solid  state,  or  from  a  gas  to  a  liquid.  Farmers  often 
protect  their  supplies  by  taking  tubs  of  water  into  their 
cellars ;  when  it  is  cold  enough  to  freeze  the  water,  the 
heat  given  off  by  the  freezing  liquid  keeps  the  cellar 
warm  enough  so  that  the  vegetables  do  not  freeze. 

Experiment  70.  —  Into  a  glass  dish  put  several  pieces  of  ice ; 
carefully  note  the  temperature  of  the  ice.  Now  apply  heat  slowly, 
testing  the  temperature  of  the  mixture  from  time  to  time  as  the 
ice  melts.  Just  before  the  last  bit  melts,  remove  the  source  of 
heat,  and  when  all  is  liquid  test  the  temperature  again.  Was  heat 
given  to  the  mass  in  the  dish  ?  How  do  you  know  this  ?  Did  this 
heat  raise  the  temperature  of  the  whole  ?  What  did  it  do 

QUESTIONS 

1.  What   are    the    common   effects   of    heat?    Does    heating 
always  produce  all  of  these  effects  in  a  body  at  the  same  time  ? 

2.  Define  expansion  and  contraction.    How  in  general  is  the 
volume  of  a  body  affected  by  heat  ?    Do  liquids  and  gases  expand 
and  contract  like  solids  ?    Name  any  uses  of  expansion  and  con- 
traction that  you  have  seen. 

3.  Why  does  ice  float  ?    Show  how  water  freezing  in  a  crevice 
may  break  a  rock.    What  is  the  rule  for  expansion  in  water  ? 

4.  Give  the  general  rule  for  changes  of  state  due  to   heat 
alone.    What  is  meant  by  the  words  liquefying,  solidifying,  and 
vaporization?    What  effect  has  pressure  upon  changes  of  state? 
Compare  the  temperature  of  water  boiling  in  a  locomotive  with 
that  of  water  boiling  in  an  open  dish. 

5.  Define  evaporation.    What  conditions  assist  evaporation? 
Why  put  your  clothing  in  a  warm  place  to  dry? 

6.  When  is  air  said  to  be  saturated  ?    If  it  is  then  cooled,  what 
happens?    How  are  clouds  formed?    What  is  a  cloud  made  up 
of  ?    Show  how  rain  is  formed  in  the  cloud  masses. 

7.  Explain  distillation.    To  what  important  uses  is  it  put  ? 


TRANSFER  OF  HEAT  79 

8.  What  is  meant  by  latent  heat  ?  What  work  is  done  by  this 
heat?  Since  the  temperature  of  ice  is  32°  F.,  and  ice  melts  at 
32°  F.,  why  does  a  block  of  ice  in  an  ice  house  remain  solid 
through  the  summer  ?  Why  does  not  ice  on  a  pond  melt  at  once 
when  the  sun  strikes  it  ?  Why  does  a  snowstorm  often  end  in 
a  storm  of  rain  ? 

SECTION   III 
TRANSFER    OF  HEAT 

89.  Methods   of   Transfer.  —  We    know   that   water 
standing  in  a  room  becomes  the  same  in  temperature  as 
the  air  around  it ;  that  if  a  warm  body  be  placed  near 
a  colder  one,  it  loses  some  of  its  heat,  while  the  other 
becomes  warmer;  that  the  earth  is  warmed  from  the 
sun ;  and  that  a  room  may  be  warmed  from  a  stove  or 
radiator,  or  a  whole  house  even  may  be  heated  from  a 
furnace   in  the   cellar.    These  things  show  that  heat 
must  be  able  to  travel  from  one  place  to  another.    Heat 
may  be  transferred  (carried  from  place  to  place)  in  three 
different  ways, — by  conduction,  radiation,  and  convection. 

90.  Conduction.  —  Conduction  is  the  transfer  of  heat 
from  one  particle  to  another  which  touches  it,  without 
change  of  relative  position  of  the  particles.    Heat  may 
flow  from  place  to  place  in  the  same  mass,  or  from  one 
body  to  another  which  touches  it,  by  conduction.    Each 
vibrating  molecule  is  supposed  to  increase  the  energy 
of  vibration  of  those  which  it  touches;  they  in  turn 
give  greater  energy  to  those  that  they  touch ;  and  so  on. 
But  each  molecule  remains  in  its  place ;  though  it  may 
vibrate    faster,    its    position  among  the   others  is  not 
changed. 


80  HEAT  AND  ENERGY 

Experiment  71.  —  Put  an  iron  rod  or  wire  in  a  hot  fire.  After 
a  few  minutes,  try  its  temperature  at  different  points,  beginning 
with  the  end  that  is  farther  from  the  fire.  Let  it  remain  and  see 
if  it  grows  hotter  throughout. 

Substances  which  allow  heat  to  pass  through  them 
easily  in  this  way  are  called  conductors  of  heat.    In  gen- 
A  eral,  solids  and  liquids  are  good  con- 

ductors as  compared  with  gases,  which 
are  very  poor.  Metals  are  usually  very 
good  conductors,  while  wood  and  cloth 
conduct  heat  but  poorly.  Stove  lifters 
and  pokers  often  have  wooden  handles, 
FIG.  59  for  this  reason ;  felt  also  is  used  around 

steam  and  water  pipes  to  keep  the  heat  in. 

Experiment  72.  —  Arrange  four  metal  wires  (e.g.  iron,  copper, 
brass,  and  German  silver),  as  in  Fig.  59.  Apply  heat  at  A,  and 
note  the  order  in  which  the  other  ends  become  hot.  Compare 
the  conducting  power  of  the  different  metals. 

Experiment  73.  —  Find  the  temperature  of  the  air  in  the  room, 
and  of  water  which  has  been  in  the  room  a  long  time  ;  they 
should  be  the  same.  Now  put  your 

hand  into  the  water.    How  does  it  feel  ?  "*?^ 

Which  takes  heat  out  of  your  hand 
faster,  air  or  water?  Which  is  the 
better  conductor? 

Experiment  74.  —  Boil  the  top  of 
water  in  a  test  tube,  as  in  Fig.  60. 
Note  how  long  it  is  before  the  bot- 
tom becomes  hot,  and  compare  with 
a  similar  length  of  iron.  Fl°-  6° 

91.  Convection.  —  Convection  is  the  transfer  of  heat 
from  place  to  place  by  the  change  of  position  of  heated 
particles.  Since  the,  molecules  of  solids  are  not  free  to 


TRANSFER  OF  HEAT 


81 


move  about,  convection  is  limited  to  liquids  and  gases. 
The  direction  of  movement  in  convection  is  upward  and 
downward,  warmer  particles  rising  and  cooler  ones 
falling.  As  any  portion  of  a  fluid  body  becomes  heated 
it  expands,  that  is,  its  particles  are  farther  apart ; 
thus  the  heated  portion  becomes  lighter  than  the  cooler 
parts  of  the  fluid  around  it.  Of  course  gravity  will  then 
pull  the  heavier  parts  downward,  and  the  lighter  heated 
portion  will  be  forced  upward. 

Experiment  75.  —  Heat  a  can  of  water.    Before  it  is  entirely 
warmed  through  test  its  temperature  at  different  depths. 

92.  Uses  of  Convection.  —  Fluids,  especially  gases,  are 
such  poor  conductors  that  they  can  only  be  heated  very 
slowly  by  conduction.  In  fact,  dry  air  is 
so  poor  a  conductor  that  it  would  hardly 
carry  heat  at  all ;  we  should  have  to  live 
constantly  in  very  cold  air,  if  it  were  not 
for  convection.  The  rise  of  heated  air 
from  a  lamp  chimney  (Fig.  61),  which 
may  be  easily  noted,  shows  us  how  readily 
air  may  be  set  in  motion ;  and  in  just  the 
same  way  the  warm  air  above  a  stove 
or  other  heater  rises  and  is  spread  about. 
Similarly  the  water  in  a  kettle  is  quickly 
heated  by  convection ;  the  warmer  parts, 
constantly  rising  to  the  top  as  they  be- 
come heated,  allow  the  colder  portions 
to  receive  heat  at  the  bottom.  Fig.  62 
shows  how  the  rise  of  warm  air  above  a  fire  keeps  it 
supplied  with  a  good  draught  of  fresh  air  from  below. 


FIG.  61 


82 


HEAT  AND  ENERGY 


Without  convection,  stoves  and  lamps  would  need  to  be 
blown  from  beneath  all  the  time.  Practically  all  winds 
are  started  by  convection,  heated  air  somewhere  being 

set   in   motion  by  cold 
air  pressing  upon  it. 

93.  Radiation.  —  Ra- 
diation is  the  transfer  of 
heat  by  vibrations  of  the 
ether.    This  statement 
may  perhaps  mean  very 
little    as   it  stands,  be- 
cause  it   brings    up    an 
idea  which  may  be  new 
to  most  of  us.     The 
transfer  of  energy  by 
as.     radiation  is  so  very  im- 
portant,   however,    that 
a  great  effort  should 
be  made  in  trying  to 
understand  it. 
First  of  all,  an  example  of  the  transfer  of  heat  by 
radiation  may  be  helpful.    If  a  fire  be  kindled  in  a  cold 
room,  objects  in  the  room  may  become  warm  some  time 
before  the  air  between  them  and  the  stove  is  equally 
heated ;  clearly  the  heat  is  not  conducted  by  the  air  to 
the  objects,  nor  does  it  travel  to  them  by  convection. 
Similarly  the  sun's  heat  warms  the  earth ;  yet  we  know 
that  the  space  between  the  sun  and  the  earth  contains 
no  matter  that  is  dense  enough  to  carry  heat  by  conduc- 
tion or  convection. 


FIG. 


TRANSFER  OF  HEAT  83 

In  order  to  explain  how  heat  can  be  thus  transferred, 
scientists  assume  that  all  space  is  filled  with  some 
medium  which  is  very  elastic,  and  they  call  it  the  ether. 
Through  this  very  elastic  medium  energy  may  travel  at 
an  exceedingly  great  speed.  The  energy  is  supposed 
to  be  transferred  by  the  vibrating  of  the  ether.  From 
its  source,  then,  energy  may  travel  through  the  ether  in 
straight  lines ;  and  this  traveling  energy  is  called  radiant 
energy  or  radiation.  It  does  not  heat  the  ether  through 
which  it  travels,  but  upon  reaching  certain  bodies  of  mat- 
ter the  radiant  energy  may  stop  and  become  changed 
into  heat  in  those  bodies,  the  degree  of  heat  being  greater 
or  less  according  to  the  nature  of  the  substance. 

We  must  understand,  of  course,  that  this  is  only  sup- 
posed to  be  the  method  by  which  heat  is  radiated.  The 
ether  is  not  a  substance  that  can  be  seen,  felt,  or 
weighed,  and  it  does  not  conform  to  our  usual  ideas  of 
matter.  Still,  scientists  are  so  very  sure  that  the  ether 
really  exists  that  we  have  come  to  accept  it  as  a  fact 
and  to  discuss  its  behavior  without  any  doubts  or  hesi- 
tation. The  source  of  heat  radiation  must  of  course  be 
a  heated  mass,  and  in  giving  off  the  radiant  energy  it 
loses  some  of  its  own  energy  or  heat. 

QUESTIONS 

1.  In  what  ways  may  heat  be  transferred? 

2.  Define  conduction.    Explain  how  heat  is  conducted  through 
a  body.    What  is  a  conductor?    Are  solids,   liquids,  and  gases 
equally  good  conductors  ?    Why  are  wooden  handles  better  than 
iron  for  stove  lifters  ? 

3.  Explain  the  cause  of  convection.    How  is  it  different  from 
conduction  ? 


84  HEAT  AND  ENERGY 

4.  Name  some  uses  of  convection.    In  what  sorts  of  matter  is 
convection  possible  V    Could  a  kettle  of  water  be  heated  if  placed 
beneath  a  fire?    Show  how  convection  is  useful  in  stoves  and 
lamps. 

5 .  How  does  radiation  differ  from  conduction  and  convection  ? 
Are  all  substances  heated  with  equal  ease  by  radiation  ?    Which  is 
warmer  in  summer,  a  tar  walk  or  a  grass  lawn  side  by  side  ?   Why  ? 

6.  Carefully  explain  radiant  energy  and  the  ether. 


SECTION   IV 
ARTIFICIAL   COLD 

94.  How  Masses  are  cooled.  —  Since  cold  means  sim- 
ply absence  of  heat,  it  follows  that  cold  cannot  be  put 
into  a  body,  but  the  only  way  to  cool  any  mass  is  to  take 
away  some  of  its  heat.    This  is  commonly  done  by  put- 
ting the  thing  near  some  cold  substance,  when  its  heat 
will  gradually  flow  into  the  other  (§  89).    In  this  way, 
things  put  into  an  ice  box  give  up  some  of  their  heat 
to  the  ice ;  thus  the  ice  is  slowly  melted  and  the  sub- 
stances become  cool.    We  feel  cold  on  a  wintry  day, 
because  heat  is  rapidly  taken  from  our  bodies  by  the 
cold  air  about  us ;  and  we  wear  clothing  not  to  keep 
out  the  cold  but  to  keep  the  heat  in. 

95.  Artificial  Cold.  —  The  common  method  of  cooling 
does  not  give  us  very  low  temperatures,  for  no  sub- 
stance is  naturally  colder  than  the  lowest  degrees  which 
climate  allows.     Very  low  temperatures  are  obtained  by 
performing  some  process  which  requires  heat,  near  the  body 
from  which  we  wish  the  heat  to  be  taken.    The  processes 
generally  used  are  melting  and  vaporization. 


ARTIFICIAL  COLD  85 

96.  Cold  by  Melting.  —  The  change  from  a  solid  to  a 
liquid  state  requires  heat.    If  it  can  be  performed  by 
some  means  other  than  directly  applying  heat,  the  sub- 
stance will  take  in  the  necessary  heat  from  wherever  it 
can  be  had.    For  example,  salt  causes  ice  to  melt,  but 
the  melting  ice  must  have  heat  in  order  to  liquefy; 
thus,  if  the  ice  and  salt  be  put  into  an  ice-cream  freezer, 
the  heat  will  be  taken  from  the  cream,  causing  it  to 
become  solid. 

Experiment  76.  —  Put  a  tablespoonful  each  of  sal  ammoniac 
and  ammonium  nitrate  (solid  salts)  into  a  tumbler  of  water.  At 
once  stir  the  whole  with  a  small  test  tube  containing  water 
(Fig.  63).  The  solid  salts  dissolve  (becoming  liquid) 
very  fast.  Does  this  process  require  heat?  From  what 
is  this  heat  taken?  With  what  result? 

97.  Cold  by  Vaporizing.  —  In  the 
same  way,  liquids  in  turning  to  gases 
take   heat  from  substances   around 
them. 

Experiment  77.  —  Pour  a  small  amount 
of  alcohol  on  the  hand,  allowing  it  to 
evaporate.  Does  it  feel  cold  ?  Blow  it,  to 

make  it  vaporize  faster.  Does  it  feel  colder  ?  Try  naphtha,  ether, 
or  chloroform  in  the  same  way.  Do  bottles  of  these  liquids  seem 
cool  to  the  touch  ? 

The  making  of  artificial  ice  depends  upon  this  princi- 
ple. Liquid  ammonia  can  be  kept  in  its  liquid  state 
only  under  great  pressure ;  as  soon  as  the  pressure  is 
removed,  the  ammonia  vaporizes  rapidly,  requiring  much 
heat.  This  is  done  near  boxes  of  water,  so  that  the  heat 
is  taken  from  the  water,  freezing  it. 


86  HEAT  AND  ENERGY 

Very  low  temperatures  are  reached  by  similar  means ; 
even  air  and  other  gases  have  been  liquefied  when  put 
under  great  pressure  and  cooled  by  other  gases  expand- 
ing around  them.  The  temperature  of  liquefied  air  is 
about  191°  C.  below  zero.  It  has  been  computed  that 
absolute  cold  (i.e.  a  condition  of  no  heat  at  all)  would  be 
reached  at  273°  C.  below  zero,  or  -  459.4°  F.  The  low- 
est degree  that  has  been  reached  is  about  —  250°  C. 

QUESTIONS 

1.  Can  cold  be  put  into  a  body?    How  may  a  substance  be 
cooled  ?    In  what  way  is  this  commonly  done  ? 

2.  Do  any  substances  naturally  have  very  low  temperatures? 
Why?    What  must  be  done  in  order  to  get  very  low  degrees? 
What  two  processes  are  commonly  used? 

3.  Explain  how  cold  is  produced  by  melting.    Show  how  this 
method  is  used  in  freezing  cream. 

4.  State  examples  of  cold  produced  by  vaporization.    How,  in 
genera],  is  artificial  ice  made? 

5.  How   are  gases   liquefied   at  low  degrees?     What  is  the 
condition  of  absolute  cold  ?    At  what  degree  would  it  be  reached  ? 

SECTION   V 
ENERGY 

98.  Transformation  of  Energy. — We  have  learned  that 
energy  is  the  ability  to  cause  motion  (§5),  and  we  know 
that  this  ability  may  be  given  from  one  body  to  another. 
For  example,  a  coiled  spring  may  lie  at  full  length  on  a 
table  with  no  ability  to  cause  motion ;  but  press  its  coils 
together  (Fig.  64)  and  it  is  then  able  to  exert  force  to 
get  back  to  its  former  length.  Energy  is  put  into  the 


ENERGY  87 

spring  from  the  muscular  energy  of  the  arm  in  pushing. 
Not  only  may  energy  be  transferred  from  one  body  to 
another,  but  one  kind  of  energy  may  be  changed  to  a  dif- 
ferent kind  either  in  the  same  body  or  in  passing  from 
one  to  another.  Thus,  muscular 
energy  in  the  arm  became  elasticity 
in  the  spring. 

Experiment  78.  —  Into  a  strong  test  tube 
put  an  inch  or  two  of  water.  Find  a  stop- 
per which  exactly  fits,  so  that  it  may  move 
up  and  down  easily  within  the  tube.  Push 
it  down  upon  the  water  lightly.  Now  heat 
the  water  slowly  and  with  caution.  As  the 
heating  continues,  what  do  you  notice  ?  What  sort  of  energy  is 
being  used?  What  is  its  effect  upon  the  water?  Is  energy 
imparted  to  the  water  ?  What  is  the  proof  of  this  ? 

Experiment  79.  —  Put  a  piece  of  zinc  into  a  test  tube  with 
hydrochloric  (muriatic)  acid.  Chemical  action  at  once  begins, 
the  zinc  acting  upon  the  acid.  Feel  of  the  tube  from  time  to 
time.  Into  what  form  of  energy  is  the  energy  of  chemical  action 
being  transformed  ? 

Many  other  experiments  and  common  happenings 
show  that  one  form  of  energy  may  be  changed  into  a 
different  form  in  the  same  or  in  another  body.  This 
change  of  energy  from  one  form  to  another  is  called 
transformation  of  energy.  The  following  principle  is 
generally  believed  by  scientists :  All  forms  of  energy 
are  so  related  that  any  kind  may  be  transformed  into  any 
other  kind.  The  study  of  these  changes  is  an  interest- 
ing and  important  part  of  physics. 

99.  Heat  as  a  Source  of  Energy.  —  The  use  of  heat,  as 
a  source  of  mechanical  motion  has  become  very  common. 


88  HEAT  AND  ENERGY 

Various  engines,  run  by  steam,  hot  air,  gas  explosions, 
and  naphtha,  which  are  now  widely  used  for  many  pur- 
poses, get  their  energy  from  heat.  In  all  these  engines 
the  force  which  finally  causes  the  motion  is  the  expansive 
force  of  some  gas  ;  but  the  energy  which  causes  the  gas 
to  expand  is  supplied  by  heat. 

100.  The  Steam  Engine.  —  It  is  of  course  well  known 
that  if  a  certain  amount  of  a  liquid  be  changed  to  a  gas, 
the  volume  of  the  gas  will  be  far  greater  than  that  of  the 
liquid.  But  if  this  change  is  made  in  a  closed  vessel, 

/ 


P 

FIG.  65 

the  gas  will  exert  great  force  in  trying  to  expand  to  its 
larger  volume.  A  steam  engine  makes  use  of  the  force 
exerted  by  steam  when  thus  trying  to  expand. 

Heat  is  applied  to  water  in  a  boiler,  changing  it  to 
steam.  This  steam  is  at  once  led  to  a  cylinder  where  it 
is  allowed  to  expand,  first  on  one  side  and  then  on  the 
other  of  a  piston,  p  (Fig.  65).  The  figure  (65)  should  be 
carefully  studied  until  the  action  of  the  engine  is  plain. 
Steam  comes  from  the  boiler  to  the  steam  chest  d  through 
a  pipe  t.  A  valve  v  moves  to  and  fro  in  d,  allowing 
the  steam  to  pass  to  the  cylinder  c,  first  to  one  end  and 
then  the  other  through  ports  a  and  b.  The  arrows  show 


ENERGY  89 

the  direction  of  flow  when  a  is  open ;  the  piston  p  is 

forced  toward   £»,  driving  out  the  steam  in  that  side 

through  an  opening  e  to  the  air  outside. 

Follow  the  motion  of  p  as  it  moves  the 

rod  r  and  turns  the  fly  wheel  f ;  notice 

how  this  causes  the  valve  v  to  move. 

When  this  valve  has  moved  to  the  posi- 

Tri-p         />/> 

tion  shown  in  Fig.  66,  steam  goes  through 

b  to  the  cylinder,  moving  the  piston  the  other  way  and 

driving  out  the  used  steam  through  a. 

101.  Other  Heat  Engines.  —  The  locomotive  is  a  steam 
engine  which  moves  itself  on  a  track ;  it  carries  a  boiler 
and  two  engines  (one  on  each  side)  all  on  one  frame. 
The  steam  turbine  contains  a  set  of  blades  similar  to  a 
water  wheel;  these  blades  are  fastened  to  a  shaft  and 
are  made  to  turn  around  by  jets  of  steam  which  strike 
them.  Grasoline  engines  explode  a  mixture  of  gasoline 
and  air;  the  energy  of  the  explosion  moves  a  piston 
which  is  joined  to  a  fly  wheel.  Naphtha  engines  burn 
naphtha,  using  that  heat  to  vaporize  other  naphtha  in 
a  coiled  tube.  This  vapor  is  allowed  to  expand  in  cyl- 
inders, so  that  the  action  is  somewhat  like  that  of  steam 
engines. 

QUESTIONS 

1.  Can  energy  be  given  from  one  body  to  another?     If  so 
given,  would  the  body  which  gave  it  still  contain  as  much  as  it 
had  before  ? 

2.  What  is  meant  by  transformation  of  energy  ?    From  what 
source  do  we  get  our  muscular  energy  ?    Can  this  energy  be  used 
so  that  we  feel  its  loss  ? 


90  HEAT  AND  ENERGY 

3.  State  examples  of  transformations  of  energy.    What  form 
of  energy  is  now  commonly  used  in  producing  motion  ? 

4.  State  the  principle  upon  which  heat  engines  are  based. 

5.  Describe    the    steam    engine,    explaining    its    manner    of 
working. 

6.  Name  other  heat  engines,  briefly  telling  how  they  apply 
heat  in  causing  motion. 

7.  What  fuels  are  commonly  used  in  these  engines  as  sources 
of  energy  ?    How  were  these  fuels  made  ?    Where  did  the  neces- 
sary energy  come  from  ?    What,  then,  is  the  great  original  source 
of  the  energy  now  commonly  used  on  earth  to  cause  motion? 


CHAPTER   V 

SOUND 

SECTION   I 

EXPLANATION   OF   SOUND 

102.  Wave  Motion.  —  There  are  two  sorts  of  motion, 
—  the  movement  of  a  body  from  one  place  to  another, 
and  motion  from  particle  to  particle  through  a  body.  The 
latter  is  called  wave  motion  or  a  wave.  The  motion  of 
any  single  particle  in  a  wave  is  called  vibration  or  vibra- 
tory motion.  As  a  wave  reaches  any  particle  in  a  body, 
c  d  e 


FIG.  67 

that  particle  is  moved  from  its  place,  giving  its  motion 
to  the  next  one  and  returning  again  to  its  position. 

Experiment  80.  —  Fasten  a  coiled  spring  (Fig.  67)  by  both  ends, 
a  and  b.  Pick  up  the  first  few  coils  and  crowd  them  together 
(see  J)  at  b ;  the  part  just  ahead  of  these  condensed  coils  is 
spread  apart,  as  in  c.  Now  let  go  the  coils.  The  condensed  part 
d  travels  quickly  toward  a,  the  separated  coils  going  ahead  (c) 
and  the  coils  behind  d  being  left  as  they  were  at  the  start,  e. 
Each  particle,  as  the  wave  goes  along,  first  moves  toward  b,  then 
toward  a,  and  finally  returns  to  its  place,  completing  one  vibration. 

The  body  through  which  a  wave  passes  is  called  the 
medium  of  the  wave.  The  vibrations  producing  a  wave 

91 


92 


SOUND 


may  be  of  different  sorts ;  in  the  spring  (Fig.  67)  the 
vibration  of  each  particle  is  parallel  to  the  direction  of 
the  wave  motion  itself,  while  Fig.  68  shows  each  particle 


FIG.  68 

vibrating  (ab)  at  right  angles  to  the  direction  of  the 
wave,  like  ripples  on  water.  A  wave  length  is  the  dis- 
tance from  one  particle  to  the  next  one  which  is  in  the 
same  state  of  vibration,  as  bd  (Fig.  69).  The  rate  of 
vibration  is  the  number  of  vibrations  which  pass  a  given 
point  in  one  second. 

103.  Definition  of  Sound.  —  We  are  familiar  with  wave 
motion  in  water ;  any  disturbance  —  the  wind,  a  pebble 
thrown  into  it,  a  moving  boat  or  animal  —  is  enough  to 
cause  ripples,  even  if  slight,  so  that  a  body  of  water  is 
rarely  free  from  waves.  In  just  the  same  way  the  air 
is  constantly  vibrating.  Any  slight  disturbance  sets  up 
wave  motion  in  the  elastic  atmosphere,  and  because 
there  are  so  many  more  disturbances  in  air  than  in 
water,  there  are  also  many  more  sorts  of  waves  all  the 
time.  Of  course  we  cannot  see  these  waves,  and  we  can 

b  d 


FIG.  69 


feel  only  the  greater  ones.  How  then  can  they  be  dis 
covered?  Nature  has  given  us  an  ear  for  that  purpose ; 
we  hear  these  waves  in  the  air,  and  we  call  the  sensation 


EXPLANATION  OF  SOUND 


93 


sound.  In  other  words,  sound  is  the  sensation  made  by 
waves  in  the  air  striking  the  ear.  In  order  to  get  used  to 
this  idea,  let  us  study  it  a  bit  further. 

104.  Sound  Waves.  —  Once  more  let  us  consider  how 
small  a  motion  in  still  water  will  cause  ripples  to  spread 
far  away  over  its  surface.  Now,  recalling  the  fact  that 
air  is  perfectly  elastic,  it  should  not  be  hard  to  see  that 
waves  may  likewise  be  caused  in  the  atmosphere  by 
the  many  motions  which  are  always  disturbing  it ;  and, 
as  in  water  these  waves  may  be  large  or  small,  so  in 
the  air  there  are  long  ones  and  short  ones,  according  to 
the  motion  which  caused  them.  Not  all  of  these  waves 
can  affect  the  ear  to  produce  sound,  some  ^ 
being  too  long  and  some  too  short.  Those  a<~ 
waves  which  can  produce  sound  in  the  ear  '  I 
are  called  sound  waves.  \ 

105.  The  Cause  of   Sound  Waves.— A 

tuning  fork  (Fig.  70)  may  be  used  in  show- 
ing how  sound  waves  are  started,  for  its 
vibrations  can  be  easily  seen. 

Experiment  81.  —  Strike  a  tuning  fork  sharply 
on   a  desk  and  at  once   look  for  any  vibrating 
(buzzing)  of  its  prongs.    Again  strike  the  fork ; 
then  hold  its  prongs  downward  so  that  they  lightly  dip  into  water. 
Do  you  see  anything  to  show  that  they  vibrate? 

Let  us  see  how  this  motion  causes  sound  waves  in 
the  air.  The  prong  /  (Fig.  70),  in  vibrating,  moves 
rapidly  to  and  fro  between  a  and  b.  As  it  moves  toward 
a  the  air  in  front  of  it  is  condensed,  but  is  quickly 
rarefied  as  the  prong  flies  toward  b.  Thus  the  prong, 


FIG.  70 


94 


SOUND 


rapidly  moving  to  and  fro,  causes  many  condensations 
and  rarefactions  to  follow  each  other  away  from  the 
prong.  These  pulses  of  air  are  sound  waves.  In  Fig.  71 
the  condensations  (A)  and  rarefactions  (B)  are  supposed 
to  be  moving  away  from  a  vibrating  bell. 

106.  Vibrating  Bodies.  —  It  will  perhaps  be  hard  to 
grasp  at  once  the  idea  of  sound  as  merely  a  sensation, 


FIG.  71 

which  reports  to  the  brain  the  vibrations  in  matter  about 
us.  One  difficulty  is  that  whereas  there  is  no  end  to 
the  sounds  we  hear,  it  is  not  often  we  can  discover  any 
vibration  in  the  body  which  caused  a  sound.  Tuning 
forks,  violin  strings,  piano  wires,  bells,  and  a  few 
others  show  vibration  plainly,  but  in  many  cases  there 
is  none  to  be  seen.  Sometimes  these  vibrations  are 


EXPLANATION  OF  SOUND  95 

largely  caused  in  the  air  itself,  as  when  a  gun  is  fired 
or  we  clap  our  hands ;  often  the  motion  is  so  great  that 
we  lose  sight  of  the  little  vibrations,  as  when  a  load  of 
rocks  is  dumped.  In  some  cases  the  motion  may  be 
felt  even  if  we  cannot  see  it. 

Experiment  82.  —  Hold  a  pan  in  the  hand  and  strike  it.  Can 
the  vibration  be  felt  ?  Strike  several  sharp  blows  on  an  iron  bar 
held  in  the  hand.  Do  you  feel  a  tingling  sensation  ?  Blow  a  horn 
and  touch  it  lightly  with  the  finger.  Sometimes  the  sound  waves 
from  a  distant  blast  or  a  heavy  cannon  will  shake  the  windows 
which  they  strike. 

At  least  we  must  understand  that  sound  occurs  only 
in  the  ear.  However  a  vibrating  body  may  arouse  sound 
waves  in  the  air,  it  is  still  nothing  but  a  vibrating  body. 
There  is  no  noise  in  a  gun  which  is  fired,  and  no  sound 
in  a  piano  —  nothing  but  motion.  To  be  sure,  we  are 
made  aware  of  the  motion  because  of  the  sound  that  it 
produces  in  the  ear ;  but  the  source  of  this  sensation  is 
still  only  a  body  in  vibrating  motion. 

QUESTIONS 

1.  What  two  sorts  of  motion  are  there ?    What  is  a  wave? 

2.  What  is  a  vibration?    Explain  how  each  particle  moves 
during  one  complete  vibration. 

3.  Define  a  medium.    Define  wave  length  and  rate  of  vibration. 

4.  Define  sound.    Of  what  use  is  this  sensation  to  us? 

5.  What  are  sound  waves  ?    What  is  the  first  cause  of  waves 
in  the  air?    Are  all  of  these  waves  alike?    Are  they  all  sound 
waves  ? 

6.  Show  how  a  vibrating  fork  may  cause  waves  in  the  atmos- 
phere.   What  property  of  air  makes  it  a  good  medium  to  carry 
sound  waves? 


96 


SOUND 


7.  Name  some  bodies  whose  vibrations  can  be  seen.    Name 
some  whose  vibrations  can  be   felt.    Why  can  we  not  see  the 
vibrations  in  all  masses  ? 

8.  Where  is  sound  located  ?    Is  there  sound  in  a  vibrating  body 
or  in  the  air  ? 

SECTION   II 
TRANSMISSION   OF  SOUND  WAVES 

107.  Different    Media Sound    waves    may    travel 

through  other  substances  than  air,  though  they  usually 
pass  through  the  air  a  short  distance  anyway,  before 
reaching  the  ear.  In  general,  solids  carry  sound  waves 


FIG.  72 

better  than  liquids,  and  liquids  better  than  gases.  That 
sounds  are  sharper  under  water  is  known  to  every  boy 
who  swims,  and  we  know  that  sound  waves  come 
through  the  iron  rails  of  a  track  much  faster  than 
through  the  air. 

Experiment  83. — Listen  at  one  end  of  a  log  or  an  iron  rail  while 
some  one  scratches  the  other  end  with  a  pin.  Do  the  same  with 
several  solids. 


TRANSMISSION  OF  SOUND  WAVES  97 

Experiment  84.  —  Punch  holes  in  the  bottom  of  two  clean  tin 
cans,  and  to  each  tie  one  end  of  a  stout  string  about  one  hundred 
feet  long.  Each  taking  a  can,  let  two  pupils  separate  until  the 
string  is  pulled  tight  (Fig.  72).  Can  you  talk  in  lower  tones 
through  the  can  than  through  the  air  ?  What  passes  along  the 
string  ? 

108.  Speed  of  Sound  Waves.  —  We  have,  perhaps,  seen 
a  man  strike  a  blow  at  a  distance  and  waited  some  time 
before    hearing   the  sound.    This   is   because    time   is 
needed  for  the  sound  waves  to  travel  through  the  air. 
Just  as  the  ripples  can  be  seen  to  move  away  from  the 
spot  where  a  pebble  is  dropped  in  still  water,  so  the 
sound  waves  in  air  move  away  from  a  vibrating  body 
at  a  speed  which  can  be  measured.    This  speed  is  a  little 
greater  in  a  warm  than  in  a  cold  atmosphere.    Through 
air  at  ordinary  temperatures  sound  waves  travel  about 
1125  feet  per  second.    A  mile  would  be  covered  in  about 
five  seconds. 

Experiment  85.  —  Stand  at  some  known  distance  (e.g.  half  a 
mile  or  more)  from  a  steam  whistle  which  is  soon  to  be  blown. 
Note  the  time  when  the  "  steam  "  appears  (using  a  stop  watch 
if  possible),  and  see  how  many  seconds  pass  before  you  hear 
the  sound.  Reduce  the  result  to  feet  per  second  and  compare 
with  the  rule. 

Note  other  similar  things  —  whistles  on  distant  trains  or  boats, 
guns  fired,  blows  struck,  or  engines  puffing.  Thunder  is  caused 
by  lightning  and  both  occur  at  the  same  instant.  Could  you  tell 
how  distant  is  the  lightning  by  hearing  its  thunder  ?  How  would 
you  do  this  ? 

109.  Reflection  ;  Echoes.  —  Roll  a  ball  against  a  board ; 
it  bounds  off  at  once.    The  ball  is  said  to  be  reflected, 
and  the  angle  at  which  it  leaves  the  board  is  the  same  as 


c 


98  SOUND 

that  at  which  it  struck.  Now  roll  it  so  as  to  strike 
exactly  at  right  angles  (as  cd  in  Fig.  73  strikes  ab),  and 
the  ball  will  be  reflected  so  as  to  come  straight  back  to 
your  hand. 

In  just  the  same  way  sound  waves  are  reflected  from 
any  building,  hill,  rock,  or  bank  of  woods  which  they 

strike.  Usually  these  reflected 
&  waves  pass  off  in  a  different  di- 
rection; but  when  they  strike 
squarely  at  right  angles,  they 
come  back  to  their  starting 
point  and  there  produce  a  faint 
sound.  This  sound  is  called 

an  echo.  Because  the  waves  have  lost  some  energy  in 
traveling,  the  echo  is  generally  weak;  if  the  air  is  too 
full  of  other  sound  waves,  these  weak  echoes  may  not 
be  heard. 

110.  Reverberation. — In  a  large   empty  hall  sound 
waves   may  be  reflected  from  wall  to  wall  in  many 
directions  at  the  same  time.    The  effect  of  this  confu- 
sion of  waves  upon  the  ear  is  not  a  distinct  echo,  but 
rather  a  roar.    Such  an  effect  is  called  reverberation.    It 
may  be  noticed    in    caves,   wells,   and    other  inclosed 
empty  spaces. 

111.  Forced  and  Sympathetic  Vibrations.  —  When  a 
vibrating  body  touches  another,   its  motion  may  start 
vibration  in  that  other.    In  some  cases  this  may  be  done 
also  ly  sound  waves,  the  waves  in  the  air  having  enough 
energy  to  arouse  vibration  in  certain  bodies  that  they 
strike.     This  sort  of  vibration  is  of  two  kinds,  —  forced 


TRANSMISSION  OF   SOUND  WAVES 


FIG.  74 


and  sympathetic.  To  understand  the  difference,  it  must 
be  known  that  every  body  lias  its  own  natural  rate  of 
vibration,  at  which  it  vibrates  when  free  to  do  so. 
When  the  motion  of  one 
body  causes  another  to  vi- 
brate at  a  rate  which  is  not  its 
own,  these  vibrations  are  said 
to  be  forced.  When  the  natu- 
ral rate  of  the  second  is  the 
same  as  that  at  which  the  first 
is  vibrating,  its  vibration  is 
said  to  be  sympathetic. 

Experiment  86.  —  Cause  a 
tuning  fork  to  vibrate.  Can  you  easily  hear  its  tone  ?  Now  strike 
it,  and  at  once  hold  it  to  a  table,  as  in  Fig.  74.  Is  the  sound  any 
louder?  The  table  is  forced  to  vibrate  by  the  motion  of  the  fork. 
Experiment  87.  —  Put  water  in  a  tall  jar  and  hold 
a  vibrating  fork  over  it,  as  in  Fig.  75.  Vary  the 
amount  of  water  till  the  sound  is  the  loudest.  What 
body  adds  its  vibrations  to  those  of  the  fork  ?  If  the 
air  column  were  in  forced  vibration,  would  its  length 
make  the  difference  that  you  now  find  ?  Is  its  vibra- 
tion forced  or  sympathetic  ? 

112.  Resonance.  — In  these  experiments  the 
sound  seemed  louder.  The  vibrations  of  the 
larger  body  were  added  to  those  of  the  fork, 
increasing  the  energy  of  the  sound  waves. 
In  such  cases  the  waves  are  said  to  be  re'en- 
forced.  The  ability  of  a  body  to  reenforce 
sound  waves  is  called  resonance,  and  the  body  itself 
is  a  resonator.  Bodies  of  this  sort  are  very  useful  and 
are  employed  particularly  in  musical  instruments. 


. 


FIG.  75 


100  SOUND 

113.  Resonators.  —  Thin    boards,    metal    tubes,    and 
columns  of  air  are  very  commonly  used  as  resonators. 
Fig.  76  shows  how  organ  pipes  make  use  of  air 
columns  as  resonators.    Air  ent'ering  an  opening 
at  the  bottom  strikes  a  reed  at  r,  making  it 
vibrate  ;  this  causes  air  in  the  pipe  to  vibrate, 
giving  a  loud  tone.    In  the  same  way  all  horns, 
cornets,  and  other  wind  instruments   are    only 
tubes  or  pipes  full  of  air;  vibration  is  caused 
by  the  lips  and  a  mouthpiece,  but  most  of  the 
sound  waves  come  from  the  tube  and  the  air 
PIG.  76     within,  which  act  as  resonators.    A  violin,  guitar, 
or  mandolin  would  be  useless  without  the  thin  wood 
body  and  the  air  it  incloses,  both  of  which  reenforce 
the  vibrations  of  the  strings.   Pianos  have  large  sound- 
ing boards  of  thin  wood  which   serve    as   resonators. 


FIG.  77 

Megaphones  (Fig.  77)  or  speaking  trumpets  partly 
reflect  the  sound  waves  which  would  otherwise  escape 
sidewise,  and  partly  serve  as  resonators  to  increase  the 
energy  of  the  waves.  By  their  use  sounds  may  be 
heard  at  much  greater  distances  from  their  sources 
than  usual ;  sailors,  firemen,  and  others  find  them  very 
necessary. 


DIFFERENT  KINDS  OF  SOUNDS  /If 


QUESTIONS 

1.  Compare  solids,    liquids,    and   gases   as   media  for   trans- 
mitting sound  waves.    State  any  common  examples  which  you 
have  noticed,  showing  how  some  substances  are  better  media 
than  others. 

2.  How  fast  does  a  wave  travel  through  air  ?    How  far  would 
a  wave  go  in  one  minute  ? 

3.  Explain  the  conditions  necessary  to  produce  an  echo.    Why 
is   an   echo   generally  heard  better  at  night?    Why  better  on 
water? 

4.  What  is  a  reverberation  ?   Why  do  we  not  hear  much  rever- 
beration in  a  hall  full  of  people  ? 

5.  How  are  forced  and  sympathetic  vibrations  caused?  Explain 
the  difference  between  them. 

6.  What  is  a  resonator?    How  do  resonators  serve  to  make 
sounds  seem  louder? 

7.  Name  different  bodies  which  may  act  as  resonators.    Show 
the  value  of  resonators  in  musical  instruments,  naming  several 
instruments  and  the  sort  of  resonator  they  use. 

8.  Explain  the  action  of  a  megaphone.    For  what  is  it  used  ? 

SECTION   III 
DIFFERENT   KINDS   OF   SOUNDS 

114.  Tones.  —  It  must  be  kept  in  mind  that  the  vibra- 
tions which  cause  sound  waves  are  not  only  very  small 
but  usually  very  rapid  —  in  some  cases  there  are  several 
thousand  every  second.  Each  of  these  vibrations  causes 
one  wave  in  the  air,  which  moves  away  so  fast  as  to  be 
1125  feet  distant  at  the  end  of  one  second.  But  as  the 
body  keeps  vibrating,  and  as  each  to-and-fro  motion 
causes  one  wave,  it  is  clear  that  at  the  end  of  one 
second  the  air  from  the  body  to  a  point  1125  feet  away 


102  SOUND 

in  every  direction  will  be  full  of  sound  waves  (see 
Fig.  71).  Others  will  follow  these  so  long  as  the  body 
vibrates. 

Now  when  the  vibration  of  the  body  is  simple,  every 
sound  wave  will  be  just  like  every  other  in  length  and 
form.  The  effect  of  these  regular  waves  upon  the  ear  is 
a  pleasing  sound  called  a  tone.  A  tone  may  then  be 
denned  as  the  effect  upon  the  ear  of  a  regular  succession 
of  like  waves. 

115.  Noises.  —  Almost  no  body  does  really  vibrate  in 
this  regular  manner,  however.    While  a  mass  may  move 
as  a  whole,  many  of  its  parts  vibrate  at  a  rate  of  their 
own.    Thus,  though  each  single  part  causes  vibrations 
that  are  regular,  many  different  sorts  of  sound  waves 
may  be  caused  by  the  different  vibrating  parts  at  the 
same  time  —  some  long  and  some  short.     The  effect 
of  these  many  different  kinds  of  waves  striking  the  ear 
together  is  a  sound  which  we  may  call  a  noise.    Note 
that  the   difference  between  tones   and  noises  is   not 
great.    Almost  no  tones  are  strictly  pure,  but  are  mixed 
with  a  few  weak  waves  that  are  not  great  enough  to 
affect  the  sound  seriously.    A  noise  may  be  considered 
as  simply  a  mixture  of  many  tones. 

116.  Differences  in  Tones.  —  Tones  may  differ  from 
each  other  in  three  ways —  in  loudness,  pitch,  and  quality. 
Since  noises  are  merely  mixtures  of  many  tones,  the  same 
fact  is  true  of  all  sounds. 

117.  Loudness.  —  The  loudness  of  a  sound  means  the 
greatness  of  the  sensation.    This  may  depend  upon  sev- 
eral things.    The  greatness  of  the  original  vibration  may 


DIFFERENT  KINDS  OF  SOUNDS  103 

affect  loudness,  and  this  may  depend  upon  the  size  of 
the  vibrating  body  or  the  energy  with  which  it  moves. 
If  the  ear  is  at  a  greater  distance  from  the  source  of 
the  waves,  the  sound  is  less  loud ;  the  kind  of  medium 
through  which  the  waves  travel  and  the  direction  of  the 
wind  both  have  an  effect  upon  the  loudness  of  sounds. 
The  size  of  the  receiver  is  also  an  important  factor; 
speaking  tubes  and  ear  trumpets  (Fig.  78)  serve  to  make 
sounds  louder  by  collecting  many  waves.  __  <  b 
On  a  similar  principle  is  the  holding  of 
the  hand  to  the  ear,  as  the  aged  often  do. 

118.  Pitch.  —  The  pitch  of  a  tone  is 
commonly  described  by  the  words  high 
or  low,  shrill  or  deep.  We  say  that  the 
pitch  of  a  woman's  voice  is  higher  than 
a  man's,  or  that  a  certain  bell  has  a 
lower  tone  than  another ;  and  we  know 
that  whatever  the  pitch  of  a  tone,  it  is 
quite  different  from  loudness. 

We  have  learned  that  sound  waves 
vary  greatly  in  length,  some  being  long 
and  others  short ;  the  vibrations  of  a  simple  pure  tone, 
however,  all  have  the  same  wave  length.  Now  since  all 
sound  waves  travel  through  the  air  at  the  same  speed, 
the  shorter  the  wave  length,  the  more  vibrations  will  pass 
a  given  point  in  a  second.  The  pitch  of  a  tone  depends 
upon  how  many  vibrations  reach  the  ear  in  a  second; 
the  greater  the  number,  the  higher  the  pitch.  Of  course 
the  length  of  a  wave  depends  upon  the  vibrations  in  the 
body  that  caused  it ;  so  we  may  get  some  idea  of  the 


104  SOUND 

source  of  a  sound  by  noting  its  pitch.  The  vibrations 
of  a  small  body  are  generally  more  rapid  than  in  a  larger 
body  of  the  same  sort,  and  the  sound  produced  has  a 
higher  pitch. 

119.  Limiting  Pitch.  —  Some  waves  are  too  long  and 
some  too  short  to  affect  the  ear  (§  104).    The  limits  within 
which  waves  may  cause  sound  vary  in  different  persons. 
Few  can  hear  sounds  lower  than  twenty  vibrations  per 
second,  or  higher  than  thirty  thousand  per  second. 

In  music,  middle  C  (C  natural)  has  264  vibrations  per 
second;  the  octave  above  (high  C)  has  twice  as  many 
(528),  and  the  octave  below  has  one  half  the  number 
(132).  A  man's  voice  can  rarely  make  a  tone  lower 
than  150  waves  per  second;  while  children  may,  in 
screaming,  reach  a  pitch  of  several  thousand  vibrations. 

120.  Quality.  —  The   first   two   features   of   sounds, 
pitch  and  loudness,  are  common  in  our  experience  and 
not  hard  to  understand ;  the  third,  quality,  may  need  a 
bit  of  thought.    A  piano  and  a  violin  may  sound  the 
same  tone ;  it  has  the  same  pitch  in  each  case,  and  may 
be  sounded  with  equal  loudness,  yet  we  should  have  no 
trouble  at  all  in  telling  the  sound  of  a  violin  from  that 
of  a  piano.    The  same  would  be  true  of  tones  made  by  a 
cornet,  banjo,  harmonica,  harp,  flute,  or  other  instrument. 
Clearly  there  is  some  feature  of  tones,  other  than  loud- 
ness  or  pitch,  which  seems  to  depend  upon  the  instru- 
ment that  produces  them ;  this  feature  is  called  quality. 

121.  Quality  explained.  —  Very  few  sounds  are  pure 
tones   (§§  114,  115);    even   those    which    are    pleasing 
enough  to  be  called  musical  contain  a  few  weaker  tones 


DIFFERENT  KINDS  OF  SOUNDS 


105 


besides  the  chief  or  fundamental  tone.  These  weaker 
ones  are  called  overtones;  their  effect  is  not  great 
enough  to  make  the  sound  unpleasant  or  to  alter  its 
pitch,  but  still  their  presence  in  the  sound  can  be  noticed 
by  the  ear.  It  is  the  effect  of  these  overtones  which 
gives  to  a  sound  its  quality.  And  since  the  overtones 
may  differ  in  different  instruments,  it  is  plain  that  one 
tone  may  be  like  another  in  pitch  and  loudness  and  still 
have  a  different  quality. 

122.  Musical  Sounds.  —  The  sounds  in  music  are  usu- 
ally tones  that  are  nearly  pure.    They  are  made  by  dif- 
ferent bodies   in   a   state   of   nearly  simple   vibration; 
strings,  air  columns,  metal  plates  and  tubes,  sheets  of 
wood  and  skins,   wires,   and  other  devices  are   used. 
Sometimes  the  tones  are    made    singly  and  often   in 
groups,  several  being  sounded  at  once.    In  such  cases  the 
tones  are  generally  of  such  pitch  that  their  waves  cause 
a  regular  movement  upon  the 

'ear,  making  a  pleasing  sound 
called  a  chord.  A  careless  ar- 
rangement of  tones  may  pro- 
duce a  jarring  sound  called  a 
discord.  A  succession  of  chords 
is  called  harmony. 

123.  The  Voice.  —  The  voice 
is  caused  by  the  vibration  of  the 
vocal  cords.    These  are  narrow 
strips  or  folds  of  membrane  (aa) 

on  either  side  of  an  opening,  b  (Fig.  79),  leading  to  the 
lungs.  Air  passing  through  6  causes  the  cords  to  vibrate. 


FIG.  79 


106  SOUND 

The  pitch  of  the  voice  is  raised  or  lowered  by  drawing 
the  cords  more  or  less  tightly ;  loudness  depends  upon  the 
energy  with  which  the  air  is  driven  out,  and  quality 
upon  the  shape  and  movements  of  the  throat  and  mouth. 
/Speech  is  made  by  movements  of  the  lips,  tongue,  palate, 
teeth,  and  other  parts  of  the  air  passages.  We  form 
words  mainly  by  varying  the  quality  of  the  voice ;  but 
among  some  nations,  like  the  Chinese,  pitch  also  is  of 
importance  in  talking. 

QUESTIONS 

1.  Define  a  tone.    Explain  the  difference  between  a  pure  tone 
and  a  mixed  noise. 

2.  In  what  ways  may  tones  differ  from  each  other  ? 

3.  What  is  meant  by  loudness?    Upon  what  different  con- 
ditions may  it  depend?    Show  how  ear  trumpets  help  to  make 
sounds  louder. 

4.  Name  some  familiar  sounds  that  have  a  high  pitch,  and 
some   having    a   low   pitch.     Upon    what   doe's   pitch    depend? 
How?    Do  short  waves  or  long  waves  have  the  higher  rate  of 
vibration? 

5.  What  is  the  lowest,  and  what  the  highest,  number  of  vibra- 
tions per  second  that  the  ear  generally  can  hear?    How  many 
vibrations  per  second  has  middle  C? 

6.  Give  an  example  of  two  tones  differing  only  in  quality. 
What  determines  the  quality  of  a  tone  or  sound?    What  is  an 
overtone  ? 

7.  In  music,  what  is  a  chord  ? 

8.  Explain  how  the  voice  is  produced.    How  is  the  pitch  of 
the  voice  varied  ?    How  do  we  vary  its  loudness  and  quality  ? 

9.  How  is  speech  effected  ?    Do  dumb  persons  have  a  voice  ? 
Why  can  they  not  talk? 

10.  Name  some  different  musical  instruments.    Classify  each 
as  a  stringed  instrument  or  a  wind  instrument,  etc. 


DIFFERENT  KINDS  OF  SOUNDS  107 

11.  In  stringed  instruments  what  is  the  original  vibrating 
body  ?    Does  this  have  to  be  reenf orced  in  any  way  ?    Explain  the 
use  of  the  head  of  a  banjo,  the  sounding  board  of  a  piano,  or 
the  body  of  a  guitar.    Why  does  a  piano  give  louder  sounds 
than  a  harp? 

12.  What  effect  upon  the  pitch  of  a  tone  do  you  produce  by 
tightening  the  string  that  caused  it  ?    How  does  the  length  of  a 
string  affect  its  tone?    Does  a  heavy  or  a  light  string  generally 
give  the  deeper  tone  ? 

13.  How  does  a  violin  player  vary  the  pitch  of  his  tones  as  he 
plays  ?    How  is  a  piano  tuned  ? 

14.  What  usually  causes  the  vibrations  in  a  wind  instrument  ? 
How  are  the  tones  usually  made  louder  ?    What  is  the  use  of  so 
much  tubing  in  a  horn  ? 


CHAPTER   VI 
LIGHT 

SECTION   I 
NATURE   OF   LIGHT 

124.  What  is  Light  ? —  We  already  know  a  few  things 
about  light,  —  that  it  may  come  from  objects  which  are 
hot,  that  it  travels  through  air  and  also  through  some 
solids   and  liquids,  that  it  may  travel  very  long  dis- 
tances, that  it  affects  the  eye  so  as  to  produce  sight, 
and  other  facts.    To  the  questions,  What  is  light?  and 
How  does  it  travel  ?  we  can  give  only  a  partial  answer. 

125.  Light  Waves. — In  the  study  of  heat  we  learned 
that  the  sun  and  other  heated  bodies  give  off  radiations, 
which  may  travel  to  a  distance  and  there  cause  heat  in 
some  substances  but  not  in  some  others.    We  also  know 
that  the  sun  may  shine  upon  different  bodies  equally, 
making  some  appear  light  and  others  dark.    And  again, 
we  have  noticed  that  the  sun  may  affect  our  skin  to 
color  it  in  summer,  may  cause  cloth  to  fade  and  paper 
to  become  yellow,  and  may  bring  about  other  changes 
that  are  chemical  in  their  nature.    So  we  see  that  the 
sun's  radiations  may  produce  three  different  effects,  — 
heat,  light,  and  chemical  changes. 

Scientists  think  that  these  radiations  (or  rays)  may 
be  all  of  the  same  sort,  differing  only  in  wave  length. 

108 


NATURE  OF  LIGHT 


109 


Different  effects  produced  by  them  vary  with  their  wave 
length  and  also  with  the  substance  upon  which  the  rays 
fall.  For  convenience,  however,  those  which  produce 
heat  in  a  body  are  called  heat  radiations ;  those  which 
cause  chemical  changes,  actinic  rays;  and  those  which 
affect  the  eye  to  produce  sight  are  called  light  waves. 

126.  Luminous  and  Illuminated  Bodies.  —  All  objects 
are  seen  by  means  of  the  light  waves  that  pass  from  them 
to  the  eye.  These  light  waves  may  have  their  origin 
(starting  point)  in  the  body  itself,  or  they  may  fall 
upon  it  from  some  outside  source  and  then  be  directed 
to  the  eye  from  that  object.  Bodies  which  give  out  light 
waves  from  themselves  are  called  luminous  ;  those  which 
give  off  only  waves  which  have  fallen  upon  them  from 
some  other  source  are  said  to  be  illuminated.  The  sun, 
lamp  flames,  glowing  coals,  the  electric  arc,  and  very 
hot  iron  are  examples  of  luminous  bodies  —  they  are 
sources  of  light  waves.  Such  bodies  may  be  seen  when 
no  other  source  of  light  is  present ;  whereas  illuminated 
objects  —  chairs,  tables,  books,  flowers,  clothing,  the 
earth,  plants,  animals,  the  moon, 
and  many  others  —  disappear  from 
sight  as  soon  as  all  sources  of  light 
waves  are  taken  away. 


127.  Rays.  —  Light  waves  start 
from  a  luminous  point,  as  o  (Fig.  80), 
md  extend  in  all  directions  in 
straight  lines.  The  straight  line 
marking  the  direction  of  any  one  wave  is  called  a  ray,  oa 
(Fig.  80).  Note  that  no  wave  ever  goes  in  a  curved  path, 


FIG. 


110  LIGHT 

so  long  as  the  medium  is  constant ;  whenever  the  direc- 
tion of  a  ray  is  changed,  it  is  sharply  broken  at  a  point 
and  passes  on  in  a  straight  line  until  again  changed. 

128.  The  Ether.  —  Light  waves  travel  long  distances, 
as  from  the  sun  and  far  more   distant  stars,  through 
space  which  we  know  to  be  rarer  than  any  vacuum  that 
man  can  make.    Clearly  no  air  is  needed  to  carry  these 
waves.    Yet  we  must  suppose  that  some  medium  is  neces- 
sary, even  though  it  may  be  very  rare;  therefore  we 
speak  of  this  medium  just  as  if  it  were  known  to  exist, 
and  call  it  the  ether  (§  93).    The  ether  is  supposed  to 
fill  all  space,  even  entering  the  pores  of  solid  matter. 
Light  waves  are  then  assumed  to  be  vibrations  of  the 
ether,  as  sound  waves  are  vibrations  of  the  air. 

129.  Speed  of   Light  Waves.  —  Through  space,  light 
waves  travel  about  186,000  miles  per  second.    This  speed 
is  so  great  that  for  all  distances  through  which  we  can 
see  on  earth,  the  waves  travel  instantly.    A  ray  of  light 
would  pass  entirely  around  the  earth  seven  times  in 
one  second;  and  rays  from  the  sun,  93,000,000  miles 
away,  reach  the  earth  in  a  little  over  eight  minutes. 

As  with  sound  waves,  the  speed  of  light  waves  varies 
in  different  media.  In  general,  rays  travel  faster  through 
a  rare  than  through  a  dense  medium. 

130.  The  Passage  of  Light  Waves Some  substances 

allow  light  waves  to  pass  freely  through  them;  glass, 
air,  and  water  are  examples.    We  can  see  through  them 
clearly,  and  they  are  said  to  be  transparent. 

Other  substances  allow  light  waves  to  pass  through, 
but  scatter  them  in  different  directions ;  ground  glass 


NATURE  OF  LIGHT 


111 


and  oiled  paper  are  examples.  Such  bodies  are  called 
translucent  ;  they  let  light  through  easily,  but  we  can- 
not see  objects  through  them. 

Opaque  bodies  are  those  through  which  light  waves 
will  not  pass  at  all.  Wood,  granite,  iron,  and  brick  are 
opaque.  Since  rays  will  not  passx  through  an  opaque 
substance,  it  is  clear  that  those  which  fall  upon  it 
must  either  be  taken  into  the  body  and  stopped  there, 
or  be  turned  off  from  its  surface.  Waves  taken  in 
by  a  body  are  said  to  be  absorbed;  when  they  are 
turned  off  from  its  surface,  they  are  said  to  be  reflected 

( 


131.  Shadows.  —  When  an  opaque  body  is  placed  so 
as  to  stop  the  waves  that  stream  from  a  luminous  source, 


FIG.  81 


it  is  said  to  cast  a  shadow.  In  other  words,  a  shadow  is 
a  space  from  which  light  waves  are  excluded  by  some 
opaque  mass.  Whenever  the  luminous  body  is  of  suffi- 
cient size,  there  will  be  a  lighter  edge  about  a  shadow, 


112 


LIGHT 


in  which  the  rays  from  some  part  of  this  luminous  body 
may  fall,  —  as  the  portions  ace1  and  bdd'  in  Fig.  81. 
A  dark  central  portion,  as  ac'd'b  in  Fig.  81,  receives  no 
rays  at  all ;  this  is  called  the  umbra  of  the  shadow.  The 
lighter  portion,  which  receives  some  (but  not  all)  of  the 
rays  from  fe,  is  called  the  penumbra. 

Darkness,  on  earth,  is  always  due  to  shadows.  Even 
the  darkness  of  night  is  caused  by  our  passing  into 
the  shadow  of  the  earth.  In  daytime  we  do  not  find 
it  dark  within  shadows  of  trees,  buildings,  or  other 
objects ;  this  is  because  sunlight  is  reflected  into  these 
spaces  from  air  particles  and  other  bodies  all  around 
them. 

132.  Reflection.  —  When  a  light  wave  strikes  any  sur- 
face and  is  turned  off  from  it,  the  wave  is  said  to  be 
reflected.  This  is  important.  It  is  due  to  reflection 
that  objects  can  be  illuminated ;  that  is,  objects  that  are 

6  ,   not  luminous  can  be 

" 

seen  by  means  of  the 
waves  which  fall 
upon  them  and  are 
reflected  to  the  eye. 

Experiment  88. — 

Stand  before  a  mirror, 
as  at  a  (Fig.  82),  hold- 
ing a  lighted  candle  in 
front  of  y.ou.  Rays  from 

the  candle  strike  the  mirror  at  right  angles,  at  &,  and  come  straight 
back  to  you.  Now  move  the  flame  toy.  Find  a  point  e  from  which 
your  eye  sees  the  reflection  of  the  flame  at  b.  Draw  lines  fb  and 
eb ;  measure  the  angles  dbf  and  cbe.  How  do  they  compare  ?  Take 
a  new  pointy  and  repeat. 


FIG.  82 


NATURE  OF  LIGHT  113 

From  this  experiment  we  may  learn  the  Law  of 
Reflection:  The  angle  at  which  rays  leave  a  surface  is 
equal  to  that  at  which  they  strike  it. 

Experiment  89.  —  Hold  a  small  mirror  before  you,  just  below 
the  eyes,  about  ten  inches  away,  with  its  glass  side  facing  away 
from  you.  In  the  other  hand  hold  another  mirror  a  few  inches 
farther  away,  an  inch  or  two  higher  than  the  first,  and  facing 
you.  Look  over  the  first  into  the  second ;  with  a  little  care  these 
may  be  so  placed  that  you  will  see  several  reflections  of  your 
eyes.  This  shows  that  reflected  waves  may  be  again  reflected 
many  times. 

133.  Reflection  from  Different  Surfaces.  —  Light  waves 
from  the  same  source  may  fall  upon  different  objects  and 
there  be  so  treated  ^ 
that  the  objects  will 
present  a  variety  of 
appearances  to  the 
eye.  For  example, 
rays  from  the  sun  may  FlG- 

fall  upon  several  bodies :  one  of  these  may  appear  to  be 
green,  another  red,  a  third  may  seem  dark,  and  still 
another  very  bright.  Yet  they  are  all  seen  by  means  of 
rays  which  come  from  one  source.  These  differences 
are  due  to  the  different  behavior  of  substances  toward 
light  waves ;  many  give  off  only  a  part  of  the  waves 
that  fall  upon  them,  and  their  appearance  to  the  eye 
depends  upon  what  waves  they  give  off. 

But  among  surfaces  which  reflect  nearly  all  of  the  rays 
that  strike  them,  there  is  still  a  difference ;  some  reflect 
regularly  and  others  reflect  only  scattered  light  waves. 
When  parallel  rays  from  an  object  cd  (Fig.  83)  strike 


114  LIGHT 

a  smooth  surface  ab,  such  as  glass,  still  water,  polished 
metals,  etc.,  each  ray  is  reflected  at  the  same  angle  as 
all  the  others,  and  thus  their  positions  among  each  other 
are  unchanged.  These  reflected  rays  would  form  an 

image  of  the  object,  as  ef. 
Fig.  84  may  serve  to  show 
how  such  rough  surfaces  as 
snow,  plaster  walls,  white 
cloth,  etc.,  may  reflect  light 
waves  which  do  not  make 
images.  Rays  from  the  ob- 
ject c'd'  strike  the  rough 
surface  a'bf ;  each  ray  is  of 

course  reflected  from  the  point  where  it  falls,  so  as  to 
obey  the  law  of  reflection ;  but  as  these  points  lie  in 
short  lines  of  many  directions,  the  rays  will  leave  the 
surface  afbr  in  various  directions,  forming  no  image. 

134.  Mirrors  and  Reflectors Mirrors  are  smooth 

surfaces  which  reflect  nearly  all  of  the  light  waves 
which  fall  upon  them.  Plane  mirrors  generally  form 
images  which  are  erect  and  like  the  object.  Standing 
before  a  mirror,  our  image  seems  to  be  twice  as  far 
away  as  the  distance  to  the  mirror;  in  other  words,  the 
image  appears  just  the  same  size  as  the  object  would 
appear  if  it  were  twice  that  distance  away. 

Rays  striking  a  curved  surface  are  reflected  as  if  they 
struck  a  plane  which  touched  the  curve  at  that  point 
only.  Thus,  in  Fig.  85,  parallel  rays  moving  as  shown 
by  arrows  are  turned  off  from  the  curved  surface  just 
as  if  they  had  struck  the  straight  lines  back  of  the 


NATURE  OF  LIGHT  115 

curve.  All  rays  parallel  to  these  three  would  be  simi- 
larly reflected  to  the  point/;  this  is  called  the  principal 
focus  of  the  mirror. 

Experiment  90.  —  Using  a  concave  (in-curving)  mirror,  reflect 
the  sun's  rays  to  a  focus.  Hold  a  narrow  strip  of  paper  in  front 
of  the  center  of  the  mirror,  and  try  to  find  this  focus  by  moving 
the  paper  back  and  forth.  When  found,  the  focus  will  appear  as 
a  small  spot  of  light,  but  very  bright.  Why  is  this  spot  so  much 
brighter  than  is  common  in  sunlight  ? 

If  now  any  luminous  source  be  placed  at  the  focus  of 
a  concave  surface,  those  of  its  waves  which  strike  the 


FIG.  85  FIG.  86 


surface  will  be  reflected  away  in  parallel  lines,  as  in 
Fig.  86.  Smooth  curved  surfaces  used  in  this  way  are 
called  reflectors;  they  are  much  used  in  locomotive 
headlights,  search  lights',  and  signal  lights.  They  catch 
many  rays  that  would  be  lost  sidewise  and  back  of  a 
light,  bending  them  all  in  the  direction  where  they  are 
needed. 

135.  Intensity  of  Illumination.  —  From  a  luminous 
source  light  waves  move  in  straight  lines  in  every 
direction.  But  it  is  evident  that  the  rays,  moving  in 
straight  lines  away  from  a  point,  must  be  spread  farther 
apart  as  the  distance  from  the  point  increases.  That  is, 


116  LIGHT 

the  total  area  covered  by  the  radiations  at  any  given 
distance  from  the  luminous  point  is  greater  at  a  greater 
distance  from  the  point.  And  as  the  same  rays  have  to 
illuminate  a  greater  area,  it  is  clear  that  the  intensity 
of  the  illumination  cannot  be  as  great.  In  other  words, 
the  farther  a  surface  is  removed  from  a  luminous  source, 
the  less  brightly  it  is  illuminated. 

Experiment  91.  —  Place  before  a  lamp  a  large  screen  of  wood 
or  cardboard  so  that  its  surface  will  be  5  inches  from  the  flame. 
At  a  point  nearest  the  flame  cut  a  hole  one  inch  square  in  the 
screen.  Now  place  another  screen  5  inches  beyond  the  first,  so 
that  the  rays  streaming  through  the  hole  will  all  strike  this 
screen.  Measure  the  illuminated  spot  and  compare  with  the  size 
of  the  opening.  The  second  screen  is  now  twice  as  far  from  the 
source  of  the  rays  as  is  the  first.  Move  it  to  a  point  three  times 
as  far  (i.e.  15  inches  from  the  flame),  and  again  four  times  as 
far.  In  each  case  measure  the  illuminated  spot,  compare  with 
the  opening,  and  note  the  brightness  of  the  illumination.  Make  a 
general  rule  to  apply. 

QUESTIONS 

1.  What  three  different  effects  are  produced  by  the  radiations 
from  the  sun?    What  do  we  call  these  radiations  when   they 
affect  the  eye,  causing  sight  ? 

2.  Do  any  other  bodies  besides  the  sun  give  off  light  waves  ? 
What  are  such  bodies  called  ?    Do  other  bodies  than  the  sun  give 
off  heat  radiations  and  actinic  rays  ?    State  any  examples  to  prove 
this. 

3.  How  are  we  able  to  see  such  bodies  as  do  not  give  out  light 
waves  of  their  own  ?    What  are  such  bodies  called  ? 

4.  What  is  a  light  ray  ?    What  sort  of  a  line  do  rays  usually 
take?    Can  a  wave  travel  in  a  curved  path?    How  then  can  the 
sun's  rays  light  a  room  into  which  they  do  not  stream  directly? 

5.  What  is  meant  by  the  ether?    Is  it  known  to  exist?    How 
does  it  compare  in  density  with  any  matter  that  we  know  ? 


REFRACTION 


117 


6.  How  fast  do  light  waves  travel  ?    How  long  time  are  the 
sun's  rays  in  coming  to  earth?    How  does  the  speed  of  light 
waves  vary  with  the  density  of  the  medium  ? 

7.  Define  a  transparent  substance  ;  a  translucent  substance  ; 
an  opaque  body.    State  examples  of  each.    What  may  become  of 
the  waves  that  fall  upon  an  opaque  body  ? 

8.  What  is  a  shadow?    How  is  a  shadow  caused ?    Name  the 
two  portions  of  a  shadow,  explaining  the  difference. 

9.  Why  is  it  dark  at  night?    Why  is  it  not  entirely  dark  in 
the  shade  in  daytime  ?    Why  is  it  not  dark  on  cloudy  days  ? 

10.  What  is  meant  by  reflection?    State  the  Law  of  Reflec- 
tion.   Why  do  light  waves  from  the  same  source  make  different 
impressions  upon  the  eye  when  reflected  from  different  things? 

11.  Explain  the  difference  between  reflection  from  smooth  and 
from  rough  surfaces.    What  sort  of  surfaces  reflect  rays  so  as  to 
form  images  ?    Give  examples. 

12.  How  are  waves  reflected  from  curved  surfaces?    What  is 
the  focus  of  a  curved  reflector  ?    Explain  the  use  of  reflectors  in 
obtaining  a  powerful  illumination. 


SECTION  II 
REFRACTION 

136.  Examples  of  Refraction.  —  "We  have  learned  that 
light  waves  travel  usually  in  straight 
lines  (§  127),  and  also  that  the  direction 
of  these  lines  may  be  changed  whenever 
a  ray  strikes  a  point  and  is  reflected 
(§132).  In  another  way  the  direction 
of  a  wave  may  be  changed,  —  when  it 
passes  from  one  medium  to  another  that 
is  denser  or  rarer.  Such  a  change  of 
direction  is  called  refraction,  and  the  wave  is  said  to 
be  refracted. 


FIG.  87 


118 


LIGHT 


Examples  of  refraction  are  common.  A  stick  thrust 
into  clear  water  often  appears  to  be  broken  at  the  point 
where  it  enters  the  water.  Objects  often  seem  irregu- 
lar when  viewed  through  window  glass,  and  the  size  of 
bodies  may  seem  greater  or  less  than 
real  when  seen  through  a  lens.  In 
such  cases  the  changes  are  due  only 
to  the  change  in  direction  of  the  light 
rays  in  passing  to  the  eye  through 
different  media. 


FIG.  88 


Experiment  92.  —  Hold  a  piece  of  thick 
glass  over  a  pencil  so  that  a  line  from  your 
eye  to  the  glass  meets  its  surface  at  an  acute  angle  (less  than  a 
right  angle).  Does  the  pencil  appear  broken  (Fig.  87)?  Now  hold 
the  glass  so  that  the  line  from  your  eye  would  meet  its  surface  at 
right  angles  (Fig.  88).  Does  the  pencil  now  appear  to  be  broken  ? 
Experiment  93.  —  Place  a  coin  in  the  bottom  of  a  dish  of 
water  (Fig.  89),  looking  at  it  as  from  e.  The  rays  are  refracted 
at  c,  so  that  the  coin  appears  to  be  at  p.  Now  look  straight  down 


FIG.  89 


FIG.  90 


upon  it  (Fig.  90),  so  that  the  rays  pe  meet  the  water  surface  at 
right  angles.  Are  the  rays  refracted?  Explain  the  reason  for 
these  results. 

137.  Refraction  defined From  these  experiments  we 

see    that   rays    travel    in    straight   lines  through    each 


REFRACTION  119 

medium,  and  that  they  are  bent  only  at  the  point  where 
they  pass  from  one  to  the  other.  Also  we  find  that  when 
the  rays  meet  the  surface  between  the  media  at  right 
angles,  they  are  not  bent  at  all  ;  the  angle  must  be 
acute  to  cause  refraction.  Putting  these  facts  together, 
we  may  say  :  Refraction  is  the  bending  of  light  rays  when 
they  pass  from  one  medium  to  another  of  different  density, 
at  an  acute  angle  to  the  surface  between  the  media. 

138.  Cause  of  Refraction.  —  To  understand  refraction, 
it  must  be  kept  in  mind  that  light  waves  travel  faster  in 
a  rare  than  in  a  dense 
medium.  In  Fig.  91 
let  abc  be  the  cross 
section  of  a  prism  of 
glass.  Rays  from  an 
object  (the  arrow) 
move  through  the 

FIG.  91 

air  with  equal  speed, 

so  that  when  the  first  one  reaches  the  glass  at  e'  the 
others  will  all  have  reached  the  line  e'e.  Now  since 
glass  is  denser  than  air,  the  first  ray  e1  will  move  only 
to  /',  while  e  (still  in  air)  moves  to  /;  thus  when  e 
gets  to  /  all  the  rays  will  have  come  to  the  line  ff. 
Through  the  glass  they  now  move  with  equal  speed, 
but  in  a  changed  direction.  The  first  ray  to  leave  the 
glass  at  m  will  now  travel  faster  than  those  still  within, 
so  that  when  the  last  one  leaves  the  glass  at  c,  the  direc- 
tion of  the  waves  will  again  be  changed.  Note  that  the 
waves  are  refracted  on  entering  and  on  leaving  the  prism, 
and  both  times  toward  its  base. 


120 


LIGHT 


FIG.  92 


139.  Lenses. —  A  lens  is  generally  a  piece  of  glass  hav- 
ing one  or  both  of  its  faces  curved ;.  its  use  is  to  refract 

light  waves  for  different 
purposes.  When  the  faces 
curve  inward  the  lens  is 
called  concave;  when  they 
curve  outward  the  lens  is 
convex.  Fig.  92  shows  both 
shapes  (in  dark  lines),  and 
it  also  shows  how  each  is  like  two  prisms  taken  together. 
Look  at  each  carefully  until  it  is  plain  that  parallel  rays 

would  spread  apart  after /\ 

leaving  a  concave  lens, 
and  would  come  together 
after  passing  through 
a  convex  lens  (Fig.  93). 

Convex  lenses  are  the  more  widely  used.    The  general 
effect  of  convex  lenses  is  to  converge  (bring  together) 

the  rays  that  pass 
through  them.  The 
point  where  such  rays 
meet  is  called  tine  focus, 
/(Fig.  93).  When  the 
rays  entering  the  lens 
areparallel  they  all  meet 
at  one  point,  which  is 
commonly  called  the 
principal  focus. 

FIG.  94  ^ 

Experiment  94. — Using 

a  convex  lens,  hold  it  so  as  to  converge  the  sun's  rays  on  a  piece 
of  paper  (Fig.  94).    Move  the  paper  nearer  or  farther  from  the 


REFRACTION  121 

glass  till  the  rays  all  fall  upon  one  small  spot.  What  do  you 
notice  regarding  this  spot?  If  the  lens  were  not  in  the  way,  how 
great  an  area  would  be  covered  by  the  rays  that  now  fall  here? 
Cause  the  spot  to  fall  on  tissue  paper  or  a  bit  of  gunpowder. 
Account  for  the  intensity  of  the  light  and  heat  at  this  point. 

140.  How  Images  are  formed.  —  When  light  waves  are 
sent  off  from  an  object,  each  point  sends  off  a  separate 
set  of  rays  which  will  be  collected  by  a  lens  and  refracted 
to  a  separate  focus. 

In  Fig.  95  let  ab  be  an  object  and  I  the  lens.  Now 
every  ray  sent  out  from  a  point  a  and  passing  through 


PIG.  95 

I  will  be  refracted  to  a  focus  a' ;  and  every  ray  from  b 
which  is  refracted,  will  come  to  a  focus  at  b'.  In  the 
same  way  rays  from  all  points  in  ab  will  be  refracted  to 
points  between  a'  and  br.  Each  focus  will  appear  the 
same,  and  will  have  the  same  position  among  the  others,  as 
the  point  in  the  object  from  which  its  rays  came.  Thus,  if 
a  screen  be  placed  so  that  these  foci  (plural  of  focus)  may 
be  formed  upon  it,  the  group  of  foci  together  will  form  an 
image  just  like  the  object. 

141.  The  Eye.  —  Just  behind  the  dark  opening  in 
the  eye  is  a  lens,  —  the  crystalline  lens.  This  forms 
images  on  a  screen  called  the  retina,  in  the  back  part 


122  LIGHT 

of  the  eyeball.  This  retina  is  made  of  nerve  fibers 
and  endings,  through  which  the  image  is  reported  to 
the  brain. 

Near  sight  is  caused  by  too  long  an  eyeball;  the 
image  is  formed  in  front  of  the  retina.  Concave  glasses 
may  correct  this  fault;  they  spread  the  rays,  making 
them  come  to  a  focus  farther  back.  Far  sight  is  due  to 
an  eyeball  that  is  too  short  ;  the  image  is  formed  behind 
the  retina.  Convex  glasses  will  bring  the  image  forward 
and  correct  the  trouble. 

142.  The  Photographic  Camera.  —  The  camera  is  a  box 
into  which  no  light  can  enter  except  through  a  lens  c 

*3-    This  lens  is 


—AAAA/WWj 
«P  just  far  enough  from 

I       c    ^     a  sensitive  plate  a  so 
e   that  the  rays  from  an 
outside  object  will  be 
brought  to  a  focus  on 


"Tr^AAAAAA 


/] 


a,  forming  an  image 
there.    The  plate  is 
FlG-96  covered  with    a    sub- 

stance upon  which  the  light  acts,  affecting  each  point 
according  to  the  amount  of  light  focused  upon  it.  Later 
the  plate  is  developed  and  fixed  in  a  dark  room,  after 
which  the  image  appears  plainly. 

143.  The  Microscope.  —  When  an  object  is  placed 
nearer  to  a  lens  than  its  principal  focus,  the  rays  that 
come  from  the  object  will  spread  apart  after  passing 
through  the  lens.  Such  rays  then  entering  the  eye 
make  the  object  seem  larger,  or  magnified. 


REFRACTION  123 

A  single  convex  lens  used  to  magnify  small  bodies  is 
called  a  simple  microscope.  A  compound  microscope  is  a 
group  of  several  convex  lenses  in  a  tube,  so  placed  that 
each  magnifies  the  image  formed  by  the  one  before  it. 
With  such  an  instrument  objects  may  seem  to  be  hun- 
dreds of  times  their  real  size. 

144.  The  Telescope.  —  A  telescope  is  a  device  for  view- 
ing distant  bodies.  It  generally  contains  two  convex 
lenses, — an  objective  o  (Fig.  97),  and  an  eyepiece  e.  The 
objective  serves  to  collect  as  many  rays  as  possible,  and 


FIG.  97 

the  eyepiece  magnifies  the  image  formed  by  the  objec- 
tive. A  few  telescopes  are  enormous.  One  in  the 
Yerkes  Observatory,  in  Wisconsin,  has  an  objective 
forty  inches  in  diameter.  Some  of  these  can  magnify 
images  over  five  thousand  times. 

o 

QUESTIONS 

1.  What  is  meant  by  refraction  ?    State  any  examples  of  refrac- 
tion that  are  familiar  to  you. 

2.  Name  the  conditions  that  are  necessary  in  order  that  a  wave 
may  be  refracted.    At  what  point  does  refraction  occur  ? 

3.  Explain  the  cause  of  refraction.    Show  why  a  wave  is  not 
refracted  when  it  meets  a  surface  at  right  angles. 

4.  What  is  a  lens?    How  \s  a  lens  related  to  a  prism  in  its 
effect  upon  light  rays?    Name  the  two  general  shapes  of  lenses. 

5.  What,   in  general,  is  the  effect  of   a  concave  lens  upon 
parallel  rays?    of  a  convex  lens? 


124  LIGHT 

6.  What   is   a   focus?    What   is  the  principal   focus?    Can 
there  be  more  than  one  focus  formed  through  a  lens?    Can  more 
than  one  principal  focus  be  formed  ? 

7.  Explain,  by  a  diagram,  how  images  are  formed  through  a 
convex  lens. 

8.  Briefly  show  how  images  are  formed  in  the  eye.  What  is  the 
cause  of  near  sight  and  of  far  sight  ?   How  may  each  be  remedied  ? 

9.  Describe  the  use  of  the  photographic  camera.    How  is  it 
usually  focused  ?  , 

10.  What  is  a  microscope  ?    How  does  a  convex  lens  magnify 
images  ?    What  is  a  compound  microscope  ? 

1 1.  State  the  use  of  a  telescope.    For  what  purpose  are  the  larger 
ones  made  ?  Are  spyglasses,  field  glasses,  opera  glasses,  etc.,  micro- 
scopes or  telescopes  ? 

SECTION  III 
COLOR 

145.  What  is  Color?  —  We  are  so  used  to  thinking  of 
color  as  a  part  or  property  of  any  substance,  that  we 
may  at  first  find  it  hard  to  understand  that  color  is  a 
property  of  light  waves. 

Experiment  95.  —  Examine  colored  pieces  of  paper  in  day- 
light ;  then  in  red,  blue,  or  other  colors  of  light.  (This  may  be 
done  by  cutting  a  small  opening  in  a  wooden  box  and  putting  a 
lamp  inside  ;  red,  blue,  or  other  colored  glass  may  be  held  over 
the  opening.  This  should  be  done  in  a  darkened  room.)  Now 
look  at  white  paper  in  daylight,  red  light,  etc.  Does  it  always 
seem  to  be  the  same  color  ?  Examine  pieces  of  dark  cloth  (dark 
blue,  green,  gray,  etc.)  in  lamplight ;  mark  each,  and  later  look 
at  them  in  daylight.  Can  you  guess  the  color  of  each  piece  cor- 
rectly by  lamplight  ? 

From  these  experiments  we  learn  that  the  color  of  a 
body  seems  to  change  when  differently  colored  waves 


COLOR  125 

fall  upon  it.  Also  we  know  that  any  illuminated  body- 
is  seen  by  means  of  such  waves  as  fall  upon  it  and  are 
reflected  to  the  eye.  Thus  it  is  clear  that  the  color  of 
any  object  depends  upon  what  waves  it  sends  to  the 
eye,  and  that  color  itself  is  a  property  of  the  light 
waves. 

*  Sound  waves,  we  recall,  vary  greatly  in  their  rate  of 
vibration,  and  the  resulting  difference  in  sounds  is 
called  pitch.  In  the  same  way  light  waves  differ 
greatly  in  rate  of  vibration;  the  effect  of  these  differ- 
ent vibration  rates  upon  the  eye  is  that  the  waves  have 
different  colors.  In  other  words,  the  color  of  a  light  wave 
depends  upon  its  rate  of  vibration.  For  example,  red 
waves  have  a  low  vibration  rate,  green  a  higher  rate, 
while  violet  waves  vibrate  about  twice  as  fast  as  red  ones. 

146.  White  Light.  —  Just  as  sound  waves  of  many 
different  vibration  rates  may  travel  together  from  a 
vibrating  body  to  the  ear,  so  light  waves  of  many  colors 
may  mix  together  in  one  beam  of 
light.  The  color  of  the  learn  as  a 
whole  might  be  very  different  from 
that  of  any  single  wave,  just  as 
the  pitch  of  a  noise  is  unlike  that 
of  any  one  tone  in  the  noise. 

Experiment  96 Paste  pieces  of  col- 
ored paper  (violet,  green,  and  red)  on  a  * 
circular  cardboard,  as  in  Fig.  98.  Loop  a  stout  string  through 
the  two  holes  near  the  center ;  twist  this  string  so  as  to  rotate  the 
card  rapidly  (any  boy  knows  how).  What  color  does  it  seem  to 
be  while  rotating?  Do  the  same  with  other  colors  —  two  or  more 
at  a  time. 


126  LIGHT 

The  sun  sends  out  waves  of  many  different  colors, 
so  many  that  we  do  not  know  the  number.  These 
many-colored  waves  unite  to  form  sunlight,  which  is 
commonly  said  to  be  white  light.  Thus  white  light  is 
a  mixture  of  many  colors. 

147.  The  Spectrum. — When  light  waves  are  refracted, 
those  that  have  the  faster  vibration  rate  are  bent  more 
than  those  having  the  slower  rate.  Thus  if  a  beam 
of  light  ac  (Fig.  99),  containing  three  colors,  —  red, 
green,  and  violet,  —  is  refracted  by  a  glass  prism,  the 

violet  waves  will  be 
bent  most,  the  green 
next,  and  the  red  least 
(§  145).    In  this  way 
g     the  single  beam   ac 
is  split  up,  its  three 
n      colors   of   waves    are 
separated,  and   each 

falls  on  a  different  spot  on  the  screen  mn.    The  appear- 
ance of  the  separated  colors  on  mn  is  called  a  spectrum. 

Experiment  97.  —  Hold  a  glass  prism  (a  cut-glass  pendant  or 
stopper)  so  as  to  refract  sunlight.  A  spectrum  will  be  formed, 
which  may  be  seen  somewhere  about  the  room.  Turn  the  prism 
till  the  spectrum  falls  upon  a  wall  or  any  surface  where  it  may 
be  easily  seen.  This  is  the  spectrum  of  white  light  (sunlight). 
How  many  colors  can  you  count?  Name  them  in  order.  Does 
each  stop  sharply,  or  gradually  shade  into  those  next  to  it  ? 

The  spectrum  of  white  light  shows  seven  distinct 
shades;  these  are  called  the  prismatic  colors.  In  order, 
they  are  red,  orange,  yellow,  green,  greenish  blue,  blue, 


COLOR  127 

and  violet.    Note  that  these  are  not  all  of  the  many  colors 
in  white  light,  but  only  the  more  important  groups. 

148.  The  Rainbow.  —  After  a  shower,  when  the.  air 
still  contains   many  heavy  clouds,   drops   of  water  in 
these  clouds  may  serve  to  refract  the  sunlight  passing 
through  them,  and  thus  separate  its  several  colors,  like 
the  prism  in  §  147.    Thus  there  will  appear  reflected  to 
the  eye  of  an  observer  a  spectrum  which  will  contain 
the  seven  prismatic  colors.    This  spectrum  usually  ap- 
pears as  an  arc  of  a  circle  low  down  in  the  sky;  it  is 
called  a  rainbow. 

149.  Color  of  Light  Waves.  —  Waves  from  a  luminous 
source  sometimes  appear  to  change  color  after  passing 
through  a  substance ;  a  chimney  of  red  glass  seems  to 
give  a  red  color  to  the  rays  from  a  lamp,  or  sunlight 
seems  blue  after  passing  through  blue  glass.    Now  the 
fact  is  not  that  color  is  given  to  the  waves,  but  rather 
that  some  of  the  waves  are  taken  from  the  learn  of  light. 
Red  glass,  for   example,   contains   a    substance   which 
absorbs  (takes  into  itself)  many  of  the  waves  that  enter 
it,  allowing  the  red  rays  to  pass  on  through ;  in  the 
same  way,  blue  glass  allows  mostly  the  blue  waves  to 
pass  through  it.    The  sun  often  seems  red  at  sunrise  or 
in  setting.    This  is  because  its  waves  have  to  pass  such 
a  long  distance  through  the  denser  and  dusty  air  near 
the  earth  that  many  of  the  shorter  waves  are  absorbed, 
leaving  mostly  the  red  ones  to  pass  entirely  through. 

150.  Colors  of  Objects.  —  The  color  of  luminous  objects 
depends  directly  upon    the   color   of   the  waves  they 
send  out. 


128  LIGHT 

Since  illuminated  objects  are  seen  only  by  the  waves 
that  they  reflect  to  the  eye  (§  126),  clearly  the  color  of 
such  bodies  will  depend  upon  (1)  the  colors  of  waves  that 
fait  upon  them,  and  (2)  which  of  these  waves  they  reflect. 
When  the  light  upon  an  illuminated  object  is  white 
light,  the  object  will  appear  white  if  all  the  waves  are 
reflected;  if  none  of  the  waves  are  reflected  from  the 
object,  but  all  are  absorbed,  it  is  said  to  be  black  ;  if  all 
the  waves  pass  through  a  body,  being  neither  reflected 
nor  absorbed,  the  substance  is  called  colorless.  In  the 
great  number  of  colored  objects,  however,  part  of  the 
waves  are  absorbed  while  others  are  reflected;  the  color 
of  such  substances  depends  upon  the  color  of  the  waves 
that  are  reflected.  f 

QUESTIONS 

1.  Is  color  a  property  of  objects  or  of  light  waves?    Why  do 
different  objects  seem  to  be  of  different  colors  ? 

2.  Upon  what  does  the  color  of  a  light  wave  depend?   Do  waves 
of  different  colors  ever  unite  in  one  beam?    What  is  white  light? 

3.  What  is  a  spectrum?    Explain  how  a  spectrum  is  formed 
when  light  waves  pass  through  a  prism. 

4.  How  many  distinct  colors  in  the  spectrum  of  white  light  ? 
Name  them  in  order.    Which   has   the  fastest  and  which  the 
slowest  vibration  rate  ? 

5.  What  is  a  rainbow  ?    How  is  a  rainbow  formed  ?    When 
and  where  do  rainbows  generally  appear? 

6.  Explain  why  light  waves  seem  to  change  color  on  passing 
through  certain  substances.    Why  does  the  sun  often  appear  red 
at  sunset?    Why  is  this  more  often  true  in  hot,  dry  weather? 

7.  Upon  what  two  factors  may  the  color  of  an  object  depend  ? 
In  sunlight,  what  objects  appear  white?   black?  colorless?  What 
objects  seem  red  ?  green  ?  blue  ? 

8.  Is  black  a  color  ?    How  are  black  objects  perceived  ? 


CHAPTER  VII 

ELECTRICITY 

SECTION  I 

THE   NATURE  OF  ELECTRICITY 

151.  Electrical  Energy.  —  The  question,  What  is  elec- 
tricity? has  never  been  fully  answered.  Many  things 
have  been  learned  about  its  behavior,  but  with  all 
their  study  men  have  never  been  able  to  discover  what 
it  is.  For  convenience  we  often  speak  of  currents 
of  "  electricity,"  charges  of  "  electricity,"  etc.,  as  though 
it  were  a  form  of  matter ;  but  though  we  may  do  this, 
it  must  be  borne  in  mind  that  we  do  not  know  anything 
which  proves  it  beyond  a  doubt. 

While  men  have  been  seeking  to  learn  the  nature  of 
electricity,  however,  they  have  discovered  many  things 
which  enable  us  to  make  it  very  useful.  Particularly 
important  is  the  fact  that  it  possesses  energy  which  may 
be  made  to  do  work  for  us  when  it  is  properly  controlled. 
In  this  study  we  shall  consider  not  so  much  the  nature 
of  electricity  as  the  means  by  which  electrical  energy  may 
be  made  useful  to  man.  We  shall  seek  to  learn  some- 
thing about  electrical  energy  along  three  general  lines, 
—  how  it  can  be  produced,  how  it  may  be  controlled, 
and  what  are  its  effects.  The  study  of  these  matters 
will  doubtless  show  us  much  that  is  of  interest  as  well 
as  of  importance. 

129 


130  ELECTRICITY 

152.  How  Electrical  Energy  is  produced.  —  We  have 
learned  that  in  order  to  produce  any  sort  of  energy  we 
must  transform  some  other  kind  of  energy  into  the  de- 
sired sort  (§  98).    In  general  we  may  say  that  the  kinds 
of  energy  commonly  transformed  into  electrical  energy 
are  heat,  chemical  energy,  and  mechanical  energy.    Elec- 
trical energy  seems  also  to  be  developed  upon  certain 
bodies  by  friction,  by  simple  contact,  and  by  dipping 
into  certain  liquids.    For  common  uses,  the  devices  for 
producing  electricity  are  usually  the  voltaic  cell  or  the 
dynamo.    These  will  be  explained  in  a  later  section. 

153.  How  Electrical  Energy  is  controlled.  —  Electrical 
energy  is  easily  controlled  because  electricity  passes 
readily  through  some  substances  and  not  through  others. 
A  body  through  which  electricity  passes  easily  is  called  a 
conductor;  one  through  which  it  passes  with  great  diffi- 
culty, or  not  at  all,  is  called  an  insulator.    It  must  be 
clear  that  if  a  conductor  is   surrounded  by  an  insu- 
lator, electricity  may  be  kept  within  the  conductor  and 
made  to  travel  long  distances  sometimes.    This  gives 
a  partial  idea  of  how  electrical  energy  is  controlled; 
a  fuller  explanation  of  conductors  and  insulators  is  made 
in  §  157. 

Among  the  better  conductors  are  metals  (copper,  zinc, 
iron,  etc.),  water,  animal  bodies,  the  earth,  and  others. 
Of  insulators,  dry  air  is  one  of  the  best;  more  than 
any  other  substance,  perhaps,  it  keeps  electricity  from 
spreading  about  where  it  would  only  be  lost  and  might 
do  injury.  Other  insulators  are  w*ood,  cloth,  rubber, 
and  glass. 


THE  NATURE   OF  ELECTRICITY  131 

154.  Electrical  Effects.  —  The  effects  produced  by 
electrical  energy  may  be  divided  into  four  classes, — 
electrolytic,  physiological,  thermal,  and  magnetic  effects. 

Briefly,  the  electrolytic  effect  is  that,  when  an  elec- 
tric current  is  passed  through  certain  compound  sub- 
stances, it  will  break  up  the  compound  into  the  simpler 
substances  that  compose  it.  This  process  is  useful 
in  chemistry  ;  it  is  used  also  in  electroplating  and  elec- 
trotyping. 

The  physiological  effects  are  those  produced  by  elec- 
tric action  upon  living  bodies  —  usually  animals.  Heavy 
currents  of  electricity  may  kill  animal  life ;  weaker  cur- 
rents are  sometimes  used  in  treating  certain  diseases. 

Thermal  effects  are  those  in  which  electricity  causes 
heat.  The  wire  through  which  a  strong  current  is  pass- 
ing may  become  very  hot,  as  seen  in  small  electric  lamps. 
Electric  heaters  and  furnaces  also  depend  upon  this 
effect. 

Perhaps  most  important  of  all  is  the  magnetic  effect. 
Wherever  electrical  energy  is  used  to  cause  motion  — 
in  motors,  cars,  elevators,  call  bells,  telegraph  and  tele- 
phone systems,  signals,  etc.  —  force  is  applied  by  means 
of  magnets ;  and  these,  of  course,  make  use  of  the 
magnetic  effect  of  electricity. 

155.  Potential.  —  In  describing  the  electrical  condi- 
tion of  a  body  the  word  potential  is  used  in  somewhat 
the  same  way  as  the  word  temperature  is  used  to  de- 
scribe its  condition  of  heat.  As  a  body  may  have  a  high 
or  low  temperature,  so  the  potential  of  a  body  is  said 
to  be  high  or  low ;  and  as  heat  passes  from  a  body  of 


132  ELECTRICITY 

high  temperature  to  one  whose  temperature  is  lower,  so 
electricity  may  pass  from  a  point  having  high  potential 
to  a  point  of  lower  potential. 

The  potential  of  two  points  is  described  as  high  and 
low,  or  positive  (+)  and  negative  (— ) ;  and  this  means 
that  their  electrical  condition  is  such  that  electricity 
would  tend  to  pass  from  one  to  the  other.  The  one 
from  which  electricity  would  pass  is  called  positive  and 
the  one  to  which  it  would  pass  is  said  to  be  negative. 
This  means  only  that  the  potential  of  the  first  is  higher 
than  that  of  the  second,  without  regard  to  any  fixed 
standard. 

156.  Electro-Motive  Force  (E.M.F.) — Different  sub- 
stances vary  in  the  ease  with  which  they  carry  elec- 
tricity, but  even  the  best  of  conductors  offer  some 
resistance  to  the  electric  current  passing  through  them. 
To  overcome  this  resistance  a  certain  sort  of  electri- 
cal "  pressure  "  is  required  ;  and  this  is  furnished  by  a 
difference  in  potential  between  two  points  in  the  con- 
ductor. If  one  point  has  a  high  potential  and  another 
point  has  a  lower  potential,  a  current  may  be  caused  to 
flow  from  the  first  point  toward  the  second.  This  may 
perhaps  be  better  understood  if  we  compare  it  with  the 
flow  of  heat  from  a  body  of  high  temperature  to  another 
of  lower  temperature. 

The  difference  in  potential  between  two  points  in  a 
conductor,  which  causes  the  current  to  flow  and  to 
overcome  resistance,  is  called  the  electro-motive  force  of 
the  current.  The  greater  the  difference  between  the 
potentials  of  two  points,  the  greater  is  the  electro-motive 


STATIC  ELECTRICITY  133 

force  of  the  current  in  the  conductor  that  connects 
them,  and  the  greater  is  the  ability  of  this  current  to 
flow  against  resistance. 

QUESTIONS 

1.  Why  is  electricity  important  to  man?    What,  in  general, 
may  be  studied  regarding  electrical  energy? 

2.  How  is  electrical  energy  made?    What  forms  of  energy  are 
commonly  used  for  the  purpose  ? 

3.  What  is  a  conductor?    What  is  an  insulator  ?    Name  exam- 
ples of  conductors  and  of  insulators.    Why  should  some  insulators 
(as  wood  or  cloth)  become  good  conductors  when  wet  ? 

4.  Show  the  use  of  conductors  and  insulators  in  controlling 
electrical  energy.    What  might  happen  if  air  were  a  good  con- 
ductor ? 

5.  Name  the  four  general  electric  effects.    Name  uses  to  which 
each  of  these  is  put. 

6.  What  is  meant  by  potential  ?    What  is  high  potential  and 
low  potential  ?    To  what  is  potential  somewhat  similar.  ? 

7.  By  what  means  is  a  current  kept  up?    Explain  fully  the 
meaning  of  electro-motive  force.    How  can  the   electro-motive 
force  of  a  current  be  increased  and  decreased? 


SECTION   II 
STATIC   ELECTRICITY 

157.  Electric  Charges.  —  Some  substances,  such  as 
glass,  resin,  silk  and  woolen  cloth,  when  rubbed  with 
other  bodies  of  a  similar  nature,  behave  in  a  peculiar 
manner,  as  we  may  have  noted.  For  example :  a  glass 
rod  rubbed  with  silk  cloth  will  pick  up  bits  of  paper; 
the  dry  hair  sometimes  seems  drawn  toward  a  rubber 
comb  that  is  run  through  it,  or  seems  inclined  to  "  stand 


134  ELECTRICITY 

on  end  "  after  being  thus  combed ;  sparks  are  sometimes 
seen  to  pass  from  fur  to  the  hand  that  is  rubbing  it.  In 
these  cases  both  the  body  that  is  rubbed  and  the  one 
that  does  the  rubbing  are  said  to  be  electrified,  or  charged 
with  electricity,  or  to  have  upon  them  a  charge. 

Experiment  98.  —  Rub  a  glass  rod  with  a  silk  cloth;  at  once 
bring  the  rod  near  small  bits  of  paper.  Quickly  do  the  same  thing 
with  the  cloth.  Repeat  the  experiment,  using  a  stick  of  sealing 
wax  and  a  woolen  cloth.  Do  you  see  evidence  that  any  of  these 
bodies  are  electrified  ? 

In  these  cases  the  charge  appears  to  be  on  the  sur- 
face of  the  charged  body,  and  only  at  the  parts  which 
were  touched  when  the  rubbing  was  done.  This  would 
be  true  when  the  substances  used  were  glass,  rubber, 
dry  wood,  silk,  paper,  sealing  wax,  sulphur,  porcelain, 
or  cloth.  If  certain  other  substances  were  used,  such 
as  the  metals,  the  charge  would  appear  not  only  at 
the  part  touched,  but  all  over  the  surface  of  the 
body.  Moreover,  if  this  body,  while  still  charged,  were 
brought  to  touch  another  body  which  was  like  it  in  this 
respect,  its  charge  would  at  once  spread  all  over  the 
other  body  as  well.  Thus  it  is  clear  that,  in  order  to 
charge  such  a  body,  we  should  first  separate  it  from 
other  bodies  by  a  substance  of  the  sort  first  described,— 
cloth,  glass,  etc. 

Substances  of  the  first  sort,  over  which  the  charge 
does  not  spread,  are  called  insulators  (§  153).  As  has 
been  said,  dry  air  is  one  of  the  most  important  of  these. 
Substances  of  the  second  sort  are  called  conductors ;  all 
the  metals,  the  earth,  animal  bodies,  and  water  contain- 
ing acids  or  salts  are  conductors.  We  have  said  (§  153) 


STATIC   ELECTRICITY 


135 


that  electricity  passes  through  a  conductor  ;  this  way  of 
speaking  is  in  common  use,  but  it  should  be  noted  that 
the  charge  is  on  the  surface  of  the  body  and  not  actually 
within  it. 

We  learn,  then,  that  the  electric  charge  upon  a  con- 
ductor moves  rapidly  and  covers  its  whole  surface,  while 
that  upon  a  "  nonconductor  "  (insulator)  remains  at  rest 
upon  the  part  where  it  was  developed.  In  this  section 
we  shall  study  the  case  of  charged  insulators, —  where 
the  charge  is  at  rest,  —  and  to  this  study  we  give  the 
name  electrostatics. 


158.  Positive  and  Negative  Charges.  —  When  bodies 
are  electrified  we  find  that  their  charges  may  be  one 
or  the  other  of  two  sorts,  which  seem 
to  have  certain  different  effects. 

Experiment  99.  —  Charge  a  glass  rod  by  rub- 
bing with  silk,  and  hang  it  by  a  silk  thread,  as 
in  Fig.  100.  Using  that  part  of  the  silk  cloth 
which  touched  the  rod,  bring  it  near  to  one  end 
of  the  latter  as  it  hangs  free  to  turn.  Carefully 
note  what  happens.  Now  charge  another  glass 
rod  in  the  same  way,  and  bring  its  charged 
portion  near  that  of  the  suspended  rod.  Note 
the  result  in  this  case.  Is  there  anything  in 
this  experiment  that  would  seem  to  show  a  dif- 
ference between  the  electrification  of  the  body 
that  is  rubbed  and  that  of  the  one  that  does  the 
rubbing?  If  you  conclude  that  the  glass  and 
the  silk  are  differently  electrified,  what  would  you  say  about  the 
behavior  of  two  unlike  charges  toward  each  other?  Supposing 
the  two  rods  to  bear  like  charges,  what  do  you  learn  about  the 
behavior  of  two  like  charges  toward  each  other? 


FIG.  100 


136  ELECTRICITY 

These  experiments  show  that  when  glass  is  charged 
by  contact  with  silk,  both  the  glass  and  the  silk  are 
electrified,  but  their  charges  are  opposite  in  effect.  It 
has  been  agreed  to  call  the  charge  upon  the  glass  posi- 
tive, and  the  charge  upon  the  silk  negative. 

The  effect  of  these  opposite  charges  upon  each  other 
is  such  that  when  the  bodies  bearing  them  are  light  and 
easily  moved,  these  bodies  will  move  toward  each  other ; 
for  example,  the  glass  and  the  silk.  But  when  two  bodies 
bearing  like  charges  are  brought  near  each  other,  these 
charges  act  so  as  to  push  the  bodies  apart ;  for  example, 
the  two  rods  of  glass.  Many  experiments  may  be  made 
with  small  electrified  bodies  to  prove  the  general  law 
that  Like  charges  repel,  and  unlike  charges  attract, 
each  other. 

The  sort  of  electrification  that  any  body  receives 
varies  according  to  its  own  nature  and  that  of  the  sub- 
stance with  which  it  is  rubbed.  Thus,  while  glass  may 
be  positively  electrified  by  rubbing  with  silk,  it  is  nega- 
tively charged  by  flannel.  Other  conditions  may  also 
affect  the  sort  of  charge  received. 

159.  Electrostatic  Induction.  —  If  a  body  that  is  not 
charged  is  brought  near  a  charged  body,  and  both  are 
separated  from  the  earth  by  insula- 
tors, the  first  shows  signs  of  being 
electrified.    In  Fig.  101,  let  a  be  an 
electrified  body  and  b  an  uncharged 
FIG.  101  k0(jy .  SUSpenci  them  by  silk  strings 

and  bring  b  near  to  a.    The  side  of  b  that  is  nearer  a  will 
receive  a  charge  of  the  opposite  kind  to  that  of  a,  while 


STATIC  ELECTRICITY 


137 


the  side  farther  from  a  will  receive  a  charge  of  the  same 
kind.  The  body  b  is  said  to  be  charged  by  induction,  and 
the  charges  which  it  receives  are  called  induced  charges. 
If  a  ball  of  pith  be  suspended  by  a  silk  thread,  as  in 
Fig.  102,  it  may  be  used  in  several  experiments  with 
electrified  bodies.  Charge  a  glass  rod  with  silk  and 
bring  it  near  the  pith 
ball ;  note  how  far  the 
rod  can  be  removed 
from  the  ball  before 
the  force  ceases  to 
make  it  move.  Note 
also  how  readily  the 
ball  moves  when  the 
rod  is  near  it.  We 
may  get  from  this  an 
idea  of  the  extent  of 
the  field  of  force  about 
a  body  bearing  even 
a  small  charge.  The 
medium  between  the  rod  and  the  ball  must  be  the  seat 
of  energy,  and  the  ball  moves  so  as  to  make  this  energy 
less.  When  the  uncharged  ball  becomes  charged  by 
induction,  the  medium  between  the  rod  and  the  ball 
must  be  considered  as  in  a  state  of  strain. 

160.  Discharges.  —  Now  if  the  body  b  is  charged  by 
induction  from  the  electrified  body  a  (Fig.  101),  it  is  clear 
that  these  two  charges  bear  some  relation  to  each  other. 
In  other  words,  if  there  is  no  change  in  the  positions 
of  the  two  bodies,  then  a  change  in  the  potential  of  the 


FIG.  102 


138  ELECTRICITY 

charge  in  a  will  cause  a  corresponding  change  in  the 
charge  of  b.  But  since  a  induces  a  negative  charge  in 
the  nearer  portion  of  6,  any  change  that  makes  the 
potential  of  a  more  positive  will  result  in  charging  that 
portion  of  b  still  more  negatively,  —  that  is,  even  more 
unlike  the  charge  in  a. 

Thus  we  see  that  a  difference  in  potential  (§  156) 
exists  between  an  inducing  charge  and  the  nearer  por- 
tion of  the  charge  that  it  induces ;  also  that  as  the 
inducing  charge  becomes  more  intense  this  difference 
becomes  greater.  Now  difference  in  potential  gives  rise 
to  electro-motive  force,  which  can  overcome  resistance; 
so  when  the  difference  between  the  potentials  of  the 
inducing  charge  and  the  induced  charge  is  great  enough 
to  overcome  the  resistance  of  the  insulator  that  separates 
these  two  charges,  a  spark  passes  between  them.  The 
path  of  this  spark  through  air  is  a  good  conductor  at 
that  instant,  and  electricity  passes  from  one  charge  to 
the  other,  making  them  of  equal  potential.  This  sudden 
and  rapid  flow  of  electricity  is  called  a  discharge.  The 
sparks  seen  when  a  cat's  fur  is  rubbed  are  caused  by 
discharges  between  the  fur  and  the  hand. 

161.  Lightning. — Lightning  is  an  electric  discharge 
similar  to  that  just  explained,  but  on  a  far  greater 
scale.  The  positively  and  negatively  charged  bodies 
are  generally  clouds,  though  in  many  cases  the  nega- 
tive charge  is  on  the  earth.  In  some  instances  the 
positive  charge  is  on  the  earth,  the  negative  charge 
being  in  the  clouds.  The  clouds  are  more  commonly 
charged  during  the  very  rapid  and  heavy  condensation 


STATIC  ELECTRICITY  139 

of  vapor  that  causes  sudden  showers.  These  heavy  rain- 
falls occur  more  often  in  the  summer,  so  that  lightning 
is  more  common  at  that  season. 

Lightning  may  be  explained  as  follows.  Suppose  a 
positively  charged  cloud  passes  near  the  earth:  build- 
ings or  the  earth  itself  may  become  charged  by  induc- 
tion, receiving  a  charge  of  the  opposite  kind  from  that 
of  the  cloud.  If  now  the  positive  charge  in  the  cloud 
increases  in  potential,  the  induced  charge  in  the  earth 
becomes  more  negative.  This  may  go  on  until  the  dif- 
ference in  potential  between  the  two  charges  is  great 
enough  to  overcome  the  resistance  of  the  air  between 
them,  and  then  a  discharge  takes  place  from  one  to  the 
other.  The  passage  of  this  discharge  through  the  air  pro- 
duces electrical  changes  which  cause  the  spark  of  flash. 

Discharges  may  pass  from  clouds  to  the  earth,  or  from 
the  earth  to  a  cloud ;  more  often  they  go  from  one  cloud 
to  another.  Objects  through  which  the  discharge  passes 
are  commonly  said  to  be  "  struck."  Tall  buildings, 
towers,  spires,  trees,  and  the  like  are  more  in  danger  of 
being  struck,  because  a  charge  induced  on  them  is 
nearer  the  cloud.  "  Heat  lightning "  is  the  reflection 
of  distant  lightning.  Thunder  is  the  sound  caused  by 
waves  of  air  set  in  vibration  by  the  discharge. 

QUESTIONS 

1 .  How  may  a  body  become  charged  with  electricity  ?    Name 
some  substances  that  can  be  charged  in  this  way.    Do  they,  in 
general,  belong  to  the  class  of  conductors  or  insulators  ? 

2.  What   name   is  commonly  given   to  the   electricity  in   a 
charge  of  this  sort?    Why  are  not  conductors  easily  charged  in 
this  way? 


140  ELECTRICITY 

3.  Name  any  cases  of  electrically  charged  bodies  that  are 
known  to  you.    Experiment  with  a  rubber  comb  and  report  the 
results. 

4.  What  two  sorts  of  charges  are  named?  How  do  two  bodies 
behave  toward  each  other  when  similarly  charged  ?    How  do  they 
act  when  their  charges  are  unlike  ? 

5.  What  is  meant  by  an  induced  charge?    How  is  a  charge, 
induced  from  one  body  to  another  ?    Does  the  inducing  charge 
lose  some  of  its  electricity  in  the  process  ? 

6.  How  does  the  induced  charge  compare  with  the  inducing 
charge  ?    State  the  condition  of  a  charge  induced  in  a  body. 

7.  Explain  the  nature  of  an  electric  discharge.    What  must 
be  the  relation  of  two  charges  before  a  discharge  can  occur  ? 

8.  Compare  the  potentials  of  the  two  charges  after  the  dis- 
charge.   What  generally  attends  a  discharge  ?    State  any  experi- 
ence that  you  have  had  with  discharges ;  any  that  you  have  seen 
or  felt. 

9.  What  is  lightning?  Between  what  bodies  does  it  usually 
take  place  ?  When,  in  the  year,  is  lightning  most  common  ?  Why  ? 

10.  Fully  explain  the  cause  of  the  discharge.     How  long  does 
it  last  ?    What  objects  are  in  most  danger  of  being  struck  ? 

11.  Explain  the  flash  of  lightning.    What  is  thunder?    Why 
is  it  not  heard  as  soon  as  the  flash  is  seen  ? 


SECTION   III 
THE  ELECTRIC  CURRENT 

162.  The  Voltaic  Cell Most  of  the  devices  by  which 

man  employs  electrical  energy  make  use  of  what  is  called 
a  current,  that  is,  a  stream  of  electricity  passing  along 
a  conductor.  This  electric  current  is  commonly  pro- 
duced in  one  of  two  ways,  —  by  a  dynamo  or  by  a  voltaic 
cell.  In  this  section  we  shall  consider  the  cell,  which 
generally  produces  weaker  currents  than  the  dynamo. 


THE  ELECTRIC   CURRENT 


141 


Experiment  100.  —  Attach  a  piece  of  copper  wire  to  a  strip  of 
zinc,  and  another  piece  to  a  strip  of  copper.  Nearly  fill  a  tumbler 
with  water  and  pour  into  it  a  little  sulphuric  acid.  Put  the  two 
metal  strips  into  the  water,  being  very  careful  that  they  do  not 
touch  each  other  at  any  point  (Fig.  103).  Now  touch  the  ends  of 
the  wires  together,  as  in  the  figure,  hold  the  wire  over  a  compass 
needle,  and  note  any  movement  of  the  needle.  This  device  is  a 
simple  cell,  which  may  produce 
a  weak  current  of  electricity. 

A  cell  consists  of  two 
different  solid  conductors 
placed  in  any  liquid  con- 
ductor (except  in  a  fused 
metal).  It  is  found  that 
when  two  such  solid  con- 
ductors are  placed  in  such 
a  liquid,  they  will  be 
charged,  but  with  a  differ- 
ence in  the  potential  of 
each.  If  the  solid  bodies 
are  of  zinc  and  copper,  as 
in  Experiment  100,  and  the  fluid  is  sulphuric  acid  in 
water,  the  potential  of  the  copper  strip  will  be  higher 
than  that  of  the  zinc. 

Now  we  have  learned  that  the  charge  upon  a  con- 
ductor covers  its  whole  surface,  and  spreads  at  once  to 
any  other  conducting  surface  which  touches  it  (§  157). 
Thus  if  the  two  strips  are  not  allowed  to  touch  each 
other,  and  a  wire  of  some  conductor  (copper)  joins  them 
outside  the  liquid,  the  charge  on  the  copper  strip  (having 
the  higher  potential)  will  at  once  discharge  along  the 
conducting  wire  to  the  zinc  strip,  which  has  the  lower 


FIG.  103 


142  ELECTRICITY 

potential.  This  discharge  along  the  wire  is  the  electric- 
current.  The  discharge  would,  of  course,  bring  the  two 
charges  to  the  same  potential ;  but  the  action  of  the  liquid 
upon  the  two  strips  is  such  that  it  renews  the  difference 
in  potential  just  as  rapidly  as  the  discharges  are  made. 
Thus  the  difference  in  potential  is  kept  up  and  the  cur- 
rent continues  to  pass  along  the  wire. 

The  two  metal  strips  are  called  plates  or  poles ;  the 
copper  plate,  having  the  higher  potential,  is  said  to  be 
positive,  while  the  zinc  plate  is  called  negative.  Note  that 
the  plates  must  not  be  connected  within  the  cell  by  any 
conductor  except  the  liquid,  nor  outside  the  cell  by 
any  conductor  but  that  through  which  it  is  intended  the 

current  shall  flow.  Other- 
wise all  or  a  part  of  the 
current  would  be  lost  to 
useful  work. 

163.  Kinds  of  Cells.  — 
Different  substances  may 
be  used  in  cells,  for  poles  or 
for  liquids.  For  the  poles, 
copper  and  zinc,  or  carbon 
and  zinc  are  most  com- 
monly used.  The  liquid  is 

generally  water,  in  which  some  acid  or  salt  has  been 
dissolved.  Sometimes  the  liquid  is  a  weak  solution  of 
sulphuric  acid;  but  as  this  destroys  the  zinc  plate  rapidly 
it  is  not  often  used  in  practice.  More  commonly  the 
liquid  is  a  solution  of  sal  ammoniac ;  cells  of  this  sort 
are  used  for  ringing  call  bells  and  for  light  work;  the 


THE  ELECTRIC   CURRENT 


143 


current   is   not   very  strong,   but   the   cells   serve  for 
months  sometimes. 

On  telegraphs  and  signal  work,  gravity  cells  are  used 
(Fig.  104).  The  plates  are  zinc  and  copper,  and  the 
liquid  is  a  solution  of  copper  sulphate  in  water.  Many 
cells  have  to  be  used  in  order  to  give  a  current  of  any 
strength,  but  they  need  little  care  except  to  be  filled 
with  water  now  and  then.  Many  so-called  "  dry  cells  '' 
are  in  use;  these  contain  a  liquid,  but  the  outside 
covering  (which  usually  forms  the  zinc  plate)  is  sealed 
so  that  the  liquid  cannot  get  out. 

164.  The  Circuit.  —  In  order  that  a  current  may  flow 
from  the  positive  to  the  negative  plate  these  must  not 


Bell 


}Push 
Button 


only  be  connected  by  a  conductor  but  there  must  be 
another  conducting  path  back  again  to  the  starting 
point.  In  other  words,  there  must  be  a  complete  path 
of  conductors  from  the  positive  plate  through  wires  or 
instruments  to  the  negative,  then  through  the  liquid  to 


144  ELECTRICITY 

the  positive  again  (Fig.  105).  This  complete  conducting 
path  is  called  the  circuit. 

If  there  is  any  break  in  a  circuit  at  any  point  from 
beginning  to  end,  no  current  will  flow  through  any  part  of 
the  circuit.  This  matter  is  of  great  importance  to  man 
in  controlling  electrical  energy.  For  example,  note  in 
Fig.  105  that  the  circuit  is  not  complete,  being  broken 
at  the  push  button  —  for  the  weak  current  will  not  go 
through  the  air  space.  The  circuit  being  broken  at  this 
one  point,  no  current  will  go  through  any  part  of  it,  and 
the  bell  will  not  ring.  But  close  the  circuit  by  press- 
ing the  button,  and  the  current  travels  at  once  through 
every  part  of  the  conducting  path  and  rings  the  bell. 

A  completed  circuit  is  said  to  be  closed,  or  made; 
when  interrupted  at  any  point  by  an  insulator,  it  is  said 
to  be  open,  or  broken. 

165.  Resistance  of  the  Circuit.  —  We  have  learned 
that  any  conductor  offers  some  resistance  to  the  pas- 
sage of  a  current  (§  156).  The  resistance  of  a  circuit  is 
divided  into  two  classes,  —  internal,  or  that  offered  by 
the  cell ;  and  external,  or  that  of  the  wires,  instruments, 
or  other  outside  conductors.  External  resistance  depends 
upon  three  things :  the  kind  of  substance,  the  length  of 
conductor,  and  its  area  of  cross  section.  Other  things 
being  equal,  the  greater  the  length  of  a  conductor,  or  the 
smaller  its  area  of  cross  section,  the  more  resistance  it  offers 
to  the  current.  Naturally  a  current  traveling  through  a 
greater  length  of  conductor  would  meet  more  resistance  ; 
and,  lengths  being  equal,  passage  along  a  small  conductor 
would,  of  course,  be  more  difficult  than  over  a  larger. 


THE  ELECTRIC  CURRENT         145 

166.  Divided  Circuits.  —  A    main    circuit    may    be 
tapped  at   different  points   by  short  branches    (called 
shunts)  which  take  the  current  to  separate  instruments. 
Each  of  these  branches  must  of  course  join  the  main 
circuit  at  some  point  farther  on,  in  order  that  the  cur- 
rent may  pass  through  it. 

When  a  current  is  divided  in  this  way,  the  amount 
of  current  that  each  branch  gets  depends  upon  its 
resistance  :  the  more  resistance  each  branch  offers,  the  less 
current  flows  through  it.  This  principle  also  is  used  in 
controlling  electrical  energy.  For  example,  suppose  it 
is  desired  to  run  a  certain  motor  at  different  speeds: 
a  device  called  a  rheostat  is  put  into  the  same  shunt 
which  runs  to  the  motor.  The  rheostat  contains  several 
coils  of  wire,  of  different  resistances,  and  a  switch  for 
making  the  current  pass  through  one  or  more  of  these 
coils  at  will.  The  more  coils  the  current  is  made  to 
pass  through,  the  greater  the  resistance  offered  by  that 
shunt,  and  consequently  the  less  current  passes  through 
it  to  the  motor.  Motormen  on  electric  cars  move  a 
switch  and  control  the  speed  of  the  car  in  this  way. 

167.  Batteries.  —  A  group  of  cells  arranged  on  one 
circuit  is  called  a  battery.    One  cell  does  not  furnish 
enough  current  to  do  very  much  work,  so  that  the  com- 
bined strength  of  two  or  more  is  generally  used. 

Cells  may  be  combined  in  two  ways,  —  in  series  or  in 
parallel.  In  parallel  arrangement  all  the  positive  plates 
are  joined  together  and  all  the  negatives  likewise ;  the 
two  sets  are  then  connected  by  a  wire.  This  arrange- 
ment decreases  internal  resistance,  but  gives  no  gain  in 


146  ELECTRICITY 

electro-motive  force.    When  cells  are  arranged  in  series, 
the  negative  plate  of  each  cell  is  joined  to  the  positive 

plate  of  'another;  between 
any  two  cells  the  wires 
may  lead  away  to  some  in- 
strument (Fig.  106).  Series 
arrangement  is  the  more  com- 
FIG.IOG  mon;  each  cell  added  to  a 

battery  gives  a  gain  of  electro-motive  force. 

168.  Uses  of  Battery  Currents.  —  Battery  currents  are 
not  powerful  enough  to  do  heavy  work.    They  are  used 
for  call  bells,  door  openers,  spark  coils  for  firing  explo- 
sives, electric  signals,  medical  batteries,  telegraph  and 
telephone  systems,  and  like  purposes. 

169.  Electrical  Measurements.  —  Quantity  of  electricity 
is  expressed  in  terms  of  coulombs.     Current  strength^  or 
greatness   of  current,    is    measured    by   amperes.     The 
electro-motive  force   of  a  current  is  expressed  in  volts, 
while  the  ohm  is  the  unit  of  resistance.    Electrical  power, 
or  the  rate  of  doing  work,  is  measured  by  watts.    One 
watt  is  the  rate  at  which  work  is  done  when  a  current  of 
one  ampere  flows  between  two  points  under  a  pressure 
of  one  volt.    The  value  of  a  watt  is  about  y^g  of  a  horse 
power  (§  68). 

QUESTIONS 

1.  What   is  a  cell?    Tell  how  a  cell  may  be  made.    Fully 
explain  its  action.    What  causes  a  current  to  flow  from  it  ?. 

2.  What    different   elements    are    sometimes    used   in   cells? 
What  liquids?    What  sort  of  cell  is  used  in  telegraph  systems, 
and  why  ? 


MAGNETISM  147 

3.  What  is  a  circuit?    If  a  circuit  is  broken  at  any  point,  what 
is  the  effect  upon  the  current  ?    Show  the  use  of  this  in  control- 
ling electrical  energy. 

4.  Distinguish  internal  and  external  resistance.    Upon  what 
does  the  resistance  of  a  conductor  depend,  and  how? 

5.  What  is  a  shunt?    When  a  current  is  divided,  how  much 
does  each  branch  receive  ?    Show  how  the  supply  of  current  to  a 
motor  may  be  controlled  by  using  resistance. 

6.  What  is  a  battery?    In  what  two  ways  may  cells  be  ar- 
ranged ?    What  is  gained  in  each  case  ? 

7.  Name  some-  common  uses  of  battery  currents. 

8.  What  is  measured  in  units  of  coulombs  ?    of  amperes  ?    of 
watts?    of  ohms?    of  volts? 

9.  Compare  the  value  of  one  watt  with  one  horse  power. 

SECTION   IV 
MAGNETISM 

170.  Magnets.  —  We  have  learned  that  one  of  the 
most  important  electrical  effects  is  the  magnetic  effect, 
and  that  it  is  owing  largely  to  this  that  electricity  can 
be  so  generally  used  to  cause  motion  (§  154).  When 
it  is  used  to  cause  motion,  the  magnetic  force  is  com- 
monly applied  by  means  of  a  magnet.  A  magnet  may  be 
described  as  a  body  which  can  attract  iron.  In  other 
words,  if  a  magnet  is  brought  near  a  bit  of  iron,  a  force 
will  act  between  them  and  cause  them  to  move  toward 
each  other,  if  they  are  free  to  move. 

Certain  other  substances  may  be  attracted  by  magnets. 
A  kind  of  iron  ore  found  in  the  earth  is  one  of  these ; 
also  steel,  cobalt,  and  nickel.  Some  substances  are 
repelled  (pushed  away)  by  a  magnet ;  for  example,  zinc 
and  bismuth.  It  has  been  found  that  the  nature  of  this 


148  ELECTRICITY 

action  (that  is,  whether  the  magnet  shall  attract  or  repel 
a  body)  varies  according  to  the  medium  in  which  the 
magnet  is  held  ;  a  body  which  is  attracted  by  a  magnet, 
while  both  are  in  the  air,  may  be  repelled  by  it  when 
they  are  in  some  other  medium. 

171.  Magnetic  Field.  — The  fact  last  stated  (§170) 
shows  that  the  magnetic  force  acts  in  the  medium  around 
the  magnet.    Simple  experiments  with  a  magnet  and  a 
compass  needle  will  show  that  this  force  can  act  through 
considerable   distances   from   the    magnet.     The  whole 
space  in  which  the  magnetic  force  may  be  felt  is  called 
the  magnetic  field.    Any  magnet,  then,  is  surrounded  by 
a  magnetic  field,  the  different  parts  of  which  have  vary- 
ing intensities  of  force. 

In  describing  this  field  we  commonly  speak  of  it  as 
containing  lines  of  magnetic  force,  or  simply  lines  of 
force.  In  that  portion  of  the  field  where  the  force  is 
most  intense  the  lines  of  force  are  most  numerous;  that 
is,  there  the  lines  are  most  densely  crowded  together. 
To  get  an  idea  of  the  arrangement  of  these  so-called 
"  lines  of  force,"  lay  a  piece  of  cardboard  upon  a  magnet 
and  sprinkle  iron  filings  over  the  card.  Try  several  dif- 
ferent positions  of  the  card  upon  the  magnet. 

172.  How  Magnets  are  made.  —  It  is  an  easy  matter 
to  make  a  magnet  of  a  piece  of  iron  or  steel.      A  body 
so  treated  is  said  to  be  magnetized.    Two  methods  are 
generally  used.    The  first  method  is  simply  to  place  the 
piece  of  iron  or  steel  near  a  magnet,  better  in  the  more 
intense  part  of  its  field.    (Compare  this,  but  do  not  con- 
fuse it,  with  §  159.) 


MAGNETISM 


149 


The  other  method  depends  upon  an  effect  of  electric 
currents,  which  may  be  shown  by  an  experiment. 

Experiment  101. — Pass  a  wire  ab  through  a  card,  as  in  Fig. 
107,  and  sprinkle  iron  filings  on  the  card.  Now  send  a  strong 
current  through  the  wire.  Note  any  change  in  the  filings;  without 
disturbing  the  card,  study  their  positions  closely;  stop  the  cur- 
rent, watching  the 
filings  carefully. 
Place  a  small  com- 
pass c  on  the  card 
and  note  any  sign 
of  a  force  acting. 


i 


I 


FIG.  107 


This  experi- 
ment shows  that 
while  a  current  is 
passing  through 
a  wire,  the  latter 
is  surrounded  by  a  field  of  force.  If  the  wire  is  covered 
by  an  insulator  and  is  coiled  around  a  bar  of  iron  or 
steel,  this  bar  will  be  magnetized  when  a  current  is 
passed  through  the  wire  coil. 

As  a  general  rule,  a  piece  of  soft  iron  remains  magnet- 
ized only  so  long  as  it  is  being  acted  upon  by  the  force 
in  the  magnetic  field ;  a  piece  of  steel,  however,  remains 
a  magnet  for  a  long  time  after  it  is  removed  from  the 
field. 

173.  Electro-Magnets.  —  A  piece  of  soft  iron  wound 
about  with  a  coil  of  insulated  wire  is  called  an  electro- 
magnet. When  a  current  flows  through  the  wire  it 
affects  the  particles  of  the  iron  in  such  a  way  as  to  make 
the  whole  bar  a  magnet.  The  intensity  of  such  a  magnet 


150  ELECTRICITY 

may  be  greatly  increased  by  winding  several  layers 
of  the  wire  around  the  iron,  like  thread  on  a  spool. 
Of  course  the  wire  must  be  insulated,  for  if  it  were  bare 
the  current  would  be  conducted  straight  across  the  coil 
without  going  through  its  many  turns. 

Experiment  102.  —  Wind  a  piece  of  soft  iron  (e.g.  a  cut  nail) 
with  insulated  wire,  as  in  Fig.  108.  Join  one  end  of  the  wire  to 
a  battery,  holding  the  other  end  in  the  hand  so  as  to  close  and 
open  the  circuit  at  will.  With  the  circuit  open,  touch  one  end  of 
the  nail  to  a  common  tack.  Can  you  lift  the 
tack  in  this  way  ?  Now  close  the  circuit  and  try 
again.  This  device  is  a  small  electro-magnet. 
What  is  needed  in  order  that  it  may  exert  mag- 
netic force  ?  Again  opening  the  circuit,  bring 
the  end  of  the  nail  down  to  within  one  sixteenth 
of  an  inch  from  the  tack  lying  on  a  desk  ;  close 
tiie  circuit,  watching  the  tack.  Lift  the  magnet 
and  tack  a  few  inches,  and  open  the  circuit.  How 
long  before  the  tack  drops  from  the  nail  ?  How 
long  a  time  is  required  for  the  soft  iron  nail  to 
become  magnetized,  and  to  lose  its  magnetism? 

Electro-magnets    may   be   made    very 

FIG.  108  r    T     i  •  . 

powerful  by  increasing  the  number  of 
turns  in  the  wire  coil  and  the  strength  of  the  current. 
Magnets  of  this  sort  are  used  in  dynamos  and  electric 
motors.  The  use  of  soft  iron  for  the  core  of  an  electro- 
magnet allows  it  to  become  magnetized  almost  instantly, 
and  it  is  demagnetized  (loses  its  magnetism)  when  the 
current  ceases  to  flow  (§  172).  For  this  reason  electro- 
magnets are  very  useful  in  all  electrical  devices  where 
motion  is  to  be  produced  at  will ;  for  example,  call 
bells,  motors,  signals,  telegraph  systems,  etc. 


MAGNETISM  151 

174.  Permanent  Magnets.  —  We   have  learned   that 
when  a  piece  of  steel  is  magnetized  it  remains  a  magnet 
after  it  is  removed  from  the  magnetic  field.    For  this 
reason  a  magnet  made  of  steel  is  called  permanent.    Of 
course  there  are  many  kinds  of  steel,  and 

these  vary  greatly  in  their  value  as  perma- 
nent magnets. 

Experiment  103.  —  Make  an  electro-magnet  as 
in  Experiment  102.  Across  one  end  of  it  draw  a 
small  piece  of  steel  (a  needle,  knife  blade,  or  steel 
pen)  several  times,  always  in  the  same  direction.  Try 
to  pick  up  small  tacks  with  this. 

Two  forms    of   permanent  magnets   are 
common,  —  the  horseshoe  (Fig.  109)  and  the        Fm- 109 
bar   magnet   (a  straight   bar  of  steel).    They  are   not 
made  nearly  as  powerful  as  some  electro-magnets  are. 
Their  use  in  small  dynamos  and  in  telephones  is  most 
important. 

175.  Magnetic  Poles.  —  In  using   magnets   we  have 
perhaps  noticed  that  the  force  seems  to  be  greatest  at 
the  ends,  while  at  the  center  none  at  all  is  felt.    For 
this  reason  many  magnets,  both  permanent  and  electro- 
magnets, are  made  in  a  horseshoe  form,  so  as  to  bring 

the  ends  near  together 
and  exert  greatest  force 
FIG.  no  at  that  point. 

Experiment  104.  —  Lay  a  bar  magnet  down  upon  a  mass  of 
iron  filings ;  lift  it  carefully  by  the  center.  Notice  the  arrange- 
ment of  the  filings  that  cling  to  i£  (Fig.  110).  Where  are  they 
most  numerous?  Where  are  they  least  in  number?  Does  the 
number  change  gradually  or  sharply? 


152  ELECTRICITY 

The  ends  of  a  magnet,  or  the  points  where  its  mag- 
netic force  seems  greatest,  are  called  its  poles.  In  any 
magnet  the  two  poles  act  differently  toward  other  mag- 
netized bodies,  so  they  are  separately  named:  one  is 
called  the  positive  (-[-)  or  north  pole,  and  the  other 
the  negative  (—)  or  south  pole.  Permanent  magnets  are 
usually  marked  by  a  line  or  groove  across  the  positive 
pole  ;  or  the  positive  pole  may  be  marked  N  and  the 
negative  S. 

It  must  be  noted  that  the  steel  or  iron  body  is  mag- 
netized not  only  at  its  poles  but  throughout  the  body ; 
the  poles  are  simply  the  parts  where  the  magnetic  force 
acts  with  the  greatest  intensity.  To  show  this  more 
clearly,  magnetize  a  steel  needle,  dip  it  in  iron  filings, 
and  note  its  middle  part.  Now  break  it  in  the  middle, 
dip  one  part  in  the  filings,  and  note  the  end  that  was  a 
portion  of  the  middle  before  you  broke  it. 

176.  Law  of  Magnets.  —  Both  poles  of  a  magnet  will 
attract  a  piece  of  iron  that  is  not  magnetized.  Toward 
a  magnetized  body,  however,  the  two  poles  act  in  an 
opposite  manner. 

Experiment  105.  —  The  positive  pole  of  a  compass  needle  is  the 
one  that  points  northward.  Secure  a  bar  magnet  whose  poles  are 
marked.  Bring  the  positive  pole  of  the  bar  near  the  +  pole  of  the 
needle  ;  note  what  happens.  Now  bring  the  same  (  +  )  pole  of 
the  bar  near  the  negative  end  of  the  needle,  while  it  is  at  rest, 
and  make  a  note  of  the  result  here.  Again,  present  the  negative 
pole  of  the  magnet  to  the  negative  end  of  the  needle,  noting  this 
result.  Finally  bring  the  negative  end  of  the  bar  to  the  positive 
pole  of  the  compass  needle,  and  observe. 

What  poles  seem  to  attract  each  other,,and  what  poles  repel? 
Sum  up  your  results  in  a  statement  of  how  the  poles  act. 


MAGNETISM  153 

Similar  experiments  with  magnets  give  the  same 
results.  The  facts  may  be  stated  briefly  in  a  Law  of 
Magnets  as  follows :  Like  poles  repel,  and  unlike  poles 
attract,  each  other. 

177.  Magnetism  of  the  Earth.  —  A  magnetized  needle 
suspended  so  as  to  swing  in  a  vertical  plane  ("  up  and 
down  ")  is  called  a  dipping  needle.    Fig.  Ill  shows  five 
different  positions  of  a  dipping  needle  placed  on  a  bar 
magnet.    Note 

that  at  the  nega- 
tive pole  of  the 
bar  the  positive 
end  of  the  needle  FlG'm 

is  down ;  note  its  other  positions  with  care,  and  if  pos- 
sible perform  the  experiment  for  yourself. 

Now  it  has  been  found  that  a  dipping  needle  carried 
north  or  south  along  any  meridian  of  the  earth  behaves 
in  much  the  same  way.  This  and  other  occurrences 
show  that  the  earth  is  a  great  magnet,  having  its  positive 
and  negative  poles  like  any  magnetized  body.  These 
magnetic  poles  are  two  points  on  its  surface,  toward 
which  the  compass  needle  points.  The  negative  mag- 
netic pole  is  northwest  of  Hudson  Bay,  about  20°  south 
of  the  north  pole.  Straight  through  the  earth  from  this 
point,  at  a  spot  in  the  Antarctic  Ocean  about  20°  north 
of  the  south  pole,  is  the  positive  magnetic  pole. 

178.  The  Compass.  —  A   magnetized   strip   of   steel, 
finely  balanced  on  a  point  so  that  it  turns  freely,  will 
be  so  acted  upon  by  the  earth's   magnetism  that  its 
positive  pole  will  point  toward  the  negative  (northerly) 


154 


ELECTRICITY 


FIG.  112 


magnetic  pole  of  the  earth,  and  its  negative  end  of 
course  toward  the  positive  magnetic  pole.  Such  a  mag- 
netized bit  of  steel  may  be  used  as  a  compass  needle. 
This  needle  is  finely  balanced, 
and  usually  swings  over  a  card 
on  which  the  "points  of  the  com- 
pass" are  marked  (Fig.  112). 

The  compass  has  long  been  a 
valuable  aid  to  sailors  and  trav- 
elers, who  often  have  little  else  to 
guide  them.  But  owing  to  the 
position  of  the  magnetic  poles 
and  variations  in  the  earth's 
magnetism  at  different  points, 
the  needle  points  to  the  true 
north  at  only  a  few  places  on  earth.  To  find  the  true 
north,  the  user  of  the  compass  has  to  know  how  far  wrong 
the  compass  direction  is  at  that  point,  and  allow  for  it. 

QUESTIONS 

1.  What  is  magnetic  force  ?    Define  magnetism. 

2.  What  is  a  magnet  ?    Name  two  kinds  of  magnets. 

3.  How  is  an  electro-magnet  made?    How  is  it  magnetized? 
Why  should  the  wire  be  insulated  ? 

4.  Name  various  uses  of  electro-magnets,  showing  why  this 
form  of  magnet  is  particularly  useful  in  each  case.    How  may  the 
power  of  an  electro-magnet  be  increased? 

5.  Of  what  are  permanent  magnets    made?    How  are    they 
magnetized  ?    Show  any  points  of  difference  between  permanent 
magnets  and  electro-magnets.    Which  form  is  most  used  in  elec- 
trical devices,  and  why  ? 

6.  What  are  the  poles  of  a  magnet  ?    How  are  they  named  ? 
Why  are  magnets  often  made  in  horseshoe  form  ? 


INDUCED   CURRENTS 


155 


7.  State  the  Law  of  Magnets.    How  do  both  poles  act  toward 
unmagnetized  iron  ? 

8.  Why  does  the  compass  needle  point  steadily  in  one  direc- 
tion?   Where   is   the   north    magnetic   pole?    Is   it   positive   or 
negative  ? 

SECTION   V 
INDUCED   CURRENTS 

179.  Induced  Electro-Motive  Force.  —  It  has  been  found 
that  currents  can  be  produced  by  means  of  a  magnet, 
and  the  electric  currents  so  made  are  called  induced 
currents.  The 
very  powerful 
currents  now  in 
common  use  are 
induced  currents, 
and  for  this  reason 
it  is  interesting 
to  learn  how  they 
are  produced. 

Experiment  106. 
—  Balance  a  thin 
card  upon  the  two 
ends  of  a  horseshoe 
magnet,  and  sprinkle 
iron  filings  evenly  over  the  card.  Note  the  positions  taken  by 
the  filings. 

The  arrangement  of  the  filings  on  the  card  shows  the 
position  of  "  lines  of  force "  in  the  field  between  the 
poles  of  the  magnet  (Fig.  113).  Now,  if  a  circuit  of 
wire  be  moved  in  such  a  magnetic  field,  a  current  will 
flow  in  this  circuit  while  there  is  a  change  in  the  number 


PIG.  us 


156  ELECTRICITY 

of  lines  of  force  cut  by  the .  circuit.  In  other  words,  so 
long  as  there  is  any  change  in  the  intensity  of  that  part 
of  the  field  which  is  within  the  circuit,  an  electro-motive 
force  (§  156)  will  be  set  up  in  the  circuit ;  and  this 
E.  M.  F.  will  vary  according  to  the  rate  of  change  in 
the  number  of  lines  of  force  which  fall  within  the  area 
of  the  circuit.  Of  course  it  is- at  once  clear  that  such 
induced  currents  will  last  but  a  moment,  unless  we  can 
arrange  to  have  the  number  of  lines  that  fall  within  the 
area  of  the  circuit  continually  changing.  This  can  be 
done  either  by  varying  the  intensity  of  the  magnetic 
field,  or  moving  the  magnet,  or  by  moving  the  wire  cir- 
cuit in  the  field.  In  practice  the  wire  circuit  is  com- 
monly made  to  rotate  in  the  field,  thus  moving  through 
a  changing  number  of  lines  of  force. 

180.  The  Dynamo.  -  -  The  dynamo  is  a  device  for 
producing  induced  currents.  From  what  we  have  just 
learned  it  is  evident  that  such  a  device  must  consist  of 
a  magnet  and  a  wire  circuit,  together  with  some  arrange- 
ment for  varying  the  number  of  lines  within  the  area  of 
the  circuit.  The  method  in  common  use  is  to  arrange 
the  wire  circuit  on  an  axle  so  that  it  can  be  rotated 
within  the  magnetic  field. 

In  the  section  of  a  dynamo,  Fig.  114,  m  is  the  mag- 
net; the  space  between  its  poles,  p  and  p\  is  crossed 
by  lines  of  magnetic  force.  The  coil  of  wire  a  (called 
the  armature]  is  made  to  turn  about  an  axle  e,  and  in 
turning  it  cuts  the  lines  of  force.  This  sets  up  a  cur- 
rent in  the  coil,  which  passes  out  through  one  brush 
b  to  the  outside  circuit  c,  supplying  instruments  on 


INDUCED  CURRENTS 


157 


the  circuit,  and  back  to  the  armature  a  through  the 
other  brush. 

The  energy  by  which  the  dynamo  is  run  is  usually 
furnished  by  a  steam  engine  or  water  power.  It  is 
applied  at  the  axle,  being  used  to  turn  the  armature. 
This  turns  at  a  high  speed,  the  strength  of  the  current 
generally  increasing  as  its  speed  is  raised. 

181.  Direction  of  the  Current. — The  whole  circuit, 
outside  and  within  the  dynamo,  is  one  continuous  path, 


FIG.  114 

the  outside  conducting  wire  being  joined  to  that  of  the 
armature  through  the  brushes  5,  b  (Fig.  114).  Now  the 
direction  of  the  current  through  this  circuit  depends 
upon  the  direction  in  which  the  armature  a  moves 
across  the  lines  of  force ;  and  since  each  part  of  a  cuts 
these  lines  in  one  direction  during  one  half  of  a  turn, 
and  in  the  opposite  direction  the  other  half,  the  current 
will  travel  through  the  circuit  first  in  one  direction  and 


158  ELECTRICITY 

then  in  the  other.  Such  a  current  is  called  an  alternat- 
ing current.  The  alternations,  or  changes  of  direction, 
occur  with  great  rapidity. 

Alternating  currents  may  be  used  for  some  purposes, 
and  their  use  is  coming  to  be  more  important.  For 
some  uses,  however,  the  current  must  be  made  to  flow 
always  in  one  direction.  This  is  done  by  a  simple 
device  on  the  armature  called  a  commutator.  The  cur- 
rent flowing  steadily  in  one  direction  is  called  a  direct 
current. 

182.  Kinds  of  Dynamos.  —  Electro-magnets  are  used  in 
dynamos   that   are   meant   to   furnish   strong   currents, 

because  they  can  be  made 
more  powerful  than  per- 
manent magnets.  The  cur- 
rent to  supply  the  coils  of 
these  electro-magnets  may 
be  taken  from  the  dynamo 
itself,  or  from  a  separate 

PIG.  115  ni     -, 

generator  called  an  exciter. 

An  exciter  is  a  small  dynamo,  the  magnets  of  which  are 
permanent.  Fig.  115  shows  a  permanent-magnet  dynamo 
whose  armature  is  turned  by  hand.  Such  devices  are 
used  in  telephones  to  ring  the  call  bell,  and  in  a  few 
other  ways.  Their  current  is  not  great,  though  stronger 
than  the  usual  battery  currents. 

183.  Uses  of  the   Current.  —  Dynamo    currents    are 
used  in  motors,  electric  lights,  furnaces,  electric  cars, 
electroplating  and  electrotyping,  and  for  other  purposes 
where  powerful  currents  are  needed.    For  lighting  and 


PLATE  IV.     AN  ALTERNATING  CURRENT  DYNAMO 


INDUCED  CURRENTS  159 

for  the  motors  of  some  electric  car  systems  alternatr 
ing  currents  are  employed.  Direct  currents  are  used  in 
electrolysis,  electroplating  and  electro  typing,  for  many 
motors,  etc. 

184.  The  Transformer The  current  that  supplies 

small  electric  lamps  in  many  houses  and  other  buildings 
has  to  be  of  high  potential  (great  electro-motive  force)  in 
order  to  travel  through  the  long  circuit.  But  such  a 
current  might  prove  danger- 
ous if  used  freely  in  houses. 
To  lessen  the  danger  and  still 
keep  up  the  flow,  a  transformer 
is  employed. 

A  transformer  consists  of 
a  coil  of  long,  fine  insulated 
wire,  surrounded  by  a  coil 
of  short,  coarse  insulated  wire 
(Fig.  116).  The  high-potential 
alternating  current  from  the 

dynamo  being  passed  through 
,       ,  «  .  FIG.  no 

the    long,   fine    wire  coil,    an 

alternating  current  is  set  up  in  the  coarser  wire  by 
induction.  This  current  has  greater  strength  than  that 
from  the  dynamo,  but  it  has  a  lower  potential.  From 
the  coarse  wire  coil,  then,  this  low-potential  current  is 
led  to  the  building  where  it  is  used.  Because  of  its 
lower  potential  it  is  less  dangerous. 

For  some  purposes  it  may  be  desired  to  change  a  cur- 
rent of  great  strength  but  low  potential  into  a  current 
of  less  strength  and  high  potential.  To  do  this  the 


160 


ELECTRICITY 


alternating  current  is  sent  through  the  coil  of  coarse 
wire ;  the  current  induced  in  the  fine  wire  coil  will 
have  less  strength  but  higher  potential  than  the  other. 

185.  The  Induction  Coil.  —  By  means  of  this  last- 
named  principle,  currents  from  batteries  are  often 
changed  into  currents  of  high  potential.  For  this  pur- 
pose an  induction  coil  is  used.  The  induction  coil  has 
two  coils  of  wire,  a  fine  and  a  coarse 
one  as  has  the  transformer,  the 
battery  current  generally  passing 
through  the  coarse  wire  a  (Fig.  117). 
This  coil  a,  through  which  the 
battery  current  passes,  is  called  the 
primary,  and  the  other  (in  which 
a  current  is  induced)  is  called  the 
secondary  coil.  Of  course  the  wire 
of  the  secondary  coil  cuts  many 
of  the  lines  in  the  magnetic  field 
around  the  primary,  and  a  current  is 
induced  in  the  secondary  whenever 
there  is  any  change  in  the  number  of 
lines  of  force  that  it  cuts  (§  179).  In 
the  transformer  this  change  is  secured  by  using  an  alter- 
nating current ;  but  since  a  battery  gives  a  direct  cur- 
rent, the  induction  coil  must  be  so  arranged  as  to 
produce  the  necessary  change  in  some  way.  This  is 
usually  done  by  a  make-and-break  piece  c  (Fig.  117), 
which  opens  and  closes  the  circuit  very  rapidly.  The 
current  thus  rapidly  flowing  and  stopping  causes  a 
change  in  the  number  of  lines  in  the  field  surrounding 


FIG.  117 


INDUCED  CURRENTS  161 

the  coarse  wire  coil;  a  current  is  induced  in  the  fine 
wire  coil  during  each  separate  change.  This  induced 
current  has  high  potential,  and  because  the  changes 
are  so  rapid  the  effect  is  nearly  that  of  a  constant 
(though  not  direct)  flow.  Coils  of  this  sort  are  used 
in  "  medical  batteries,"  Rontgen  ray  apparatus,  wireless 
telegraphy,  etc. 

QUESTIONS 

1.  How,  in  general,  are  induced  currents  produced?  How  does 
the  electro-motive  force  of  these  currents  vary? 

2.  What  is  a  dynamo  ?    What  are  its  important  parts  ? 

3.  How  is  the  energy  for  running  a  dynamo  applied?    In  what 
part  of  the  dynamo  is  the  current  produced  ?   How  is  the  current 
taken  from  the  armature  to  the  outside  part  of  the  circuit  ? 

4.  What  is  an  alternating  current?  a  direct  current?    Why 
should  a  dynamo  current  alternate  ?    Can  such  a  current  be  used  ? 
How  is  it  changed  to  a  direct  current  ? 

5.  What  advantage  have  electro-magnet  dynamos  over  those 
using  permanent  magnets  ?    What  advantage  have  the  latter  over 
the  former  ? 

6.  Name  uses  of  currents  from  electro-magnet  dynamos.    For 
what  are  permanent-magnet  dynamos  used  ?    How  do  the  coils  of 
the  electro-magnets  in  a  dynamo  receive  their  current? 

7.  What  is  a  transformer  ?    Describe  its  structure.    For  what 
purposes  are  transformers  used  ? 

8.  Through  which  coil  in  a  transformer  would  you  pass  a  cur- 
rent that  was  to  be  changed  to  a  higher  potential?  to  a  lower 
potential  ?    What  sort  of  a  current  is  used  in  either  case  ? 

9.  Describe  the  induction  coil.    For  what  purpose  is  it  gener- 
ally used  ?    Show  how  an  induction  coil  may  use  a  direct  current, 
as  the  transformer  uses  an  alternating  current. 

10.  In  which  of  the  two  coils  is  the  current  induced  ?  State 
carefully  the  condition  under  which  a  current  will  be  induced  in 
this  coil. 


162 


ELECTRICITY 


SECTION    VI 
USES   OF  ELECTRICAL  ENERGY 

186.  Electric  Motor.  —  The  parts  of  an  electric  motor 
are  the  same  as  those  of  the  dynamo  (§  180) ;  in  fact, 
an  ordinary  dynamo  could  be  made  to  serve  as  a  motor. 
The  difference  is  that  whereas  the  armature  of  a  dynamo 
is  turned  by  some  outside  means  and  a  current  is  gen- 
erated in  it,  the  armature  of  a  motor  receives  a  cur- 
rent from  some  outside  source  and,  being  turned  by 

its  action,  is  able 
to  impart  motion  to 
other  bodies. 

Fig.  118  may 
serve  to  show  the 
action  of  a  motor. 
The  armature  ab  is 
to  turn  about  its  axle,  between  the  poles  (+  and  — ) 
of  the  magnet.  A  current  from  a  dynamo  or  battery 
enters  through  the  brush  m,  travels  around  the  arma- 
ture coil  ab,  and  out  through  n,  as  shown  by  arrows. 
Passing  through  it  in  this  way,  the  current  makes  an 
electro-magnet  of  the  armature,  its  negative  pole  being 
above  and  its  positive  pole  below  the  horizontal  position 
(dotted  lines).  That  is,  the  part  a  becomes  a  —  pole 
and  the  part  b  a  4-  pole.  Now  since  like  poles  repel  and 
unlike  attract  each  other,  a  is  repelled  by  the  —  pole 
of  the  magnet  and  attracted  by  the  -f  pole ;  also  b  is 
repelled  by  +  and  attracted  by  — .  All  of  these  forces 
tend  to  make  the  armature  turn  about  its  axle  in  the 
direction  of  the  curved  arrow.  When  it  has  turned  so 


FIG.  118 


USES  OF  ELECTRICAL  ENERGY 


163 


that  a  is  below  and  b  above  the  horizontal  position,  a 
commutator  c  changes  the  direction  of  the  current  in  the 
coil,  so  that  b  becomes  the  negative  and  a  the  positive 
pole  of  the  armature.  Thus  the  motion  goes  on  always 
in  the  same  direction. 

Such  a  motor  would  use  a  direct  current.  Motors 
are  now  commonly  made  without  commutators,  to  use 
an  alternating  current.  These  are  often  built  upon  the 
same  frame  as  the  dynamo  which  furnishes  the  current. 

187.  Electric  Cars.  —  An  electric  car  is  driven  by 
means  of  a  motor.  This  is  on  the  under  side  rf  the  car 


FIG.  119 


m  (Fig.  119) ;  as  its  armature  turns,  the  motion  is  trans- 
mitted to  the  wheels  by  a  set  of  gear  wheels  (§  72). 
The  current  is  supplied  from  a  wire  w,  or  from  a  "  third 
rail"  beneath  the  car.  After  passing  through  the  con- 
troller s  to  the  motor,  the  current  leaves  through  the 
wheels,  traveling  through  the  rails  back  to  the  dynamo. 


164 


ELECTRICITY 


The  speed  of  the  motor  and  car  is  governed  by  adding 
more  or  less  resistance  to  the  shunt  at  the  controller  * 
(§  166). 

188.  The  Telephone.  —  Only  a  very  general  explana- 
tion of  the  telephone  can  be  given  here.  In  Fig.  120, 
suppose  some  one  talking  at  A  to  a  person  at  B.  The 
two  instruments,  at  A  and  B,  may  be  alike,  though  they 
usually  differ  in  appearance.  In  each,  m  is  a  permanent 


c  m 


FIG.  120 

magnet  and  c  a  coil  of  wire  which  is  continuous  with  the 
circuit ;  d  is  a  disk  of  thin  iron. 

As  you  talk  before  0,  the  sound  waves  cause  the  disk 
d  to  vibrate.  The  disk  vibrating  near  the  magnet  in- 
duces alternate  currents  in  the  coil  c  —  in  one  direc- 
tion when  d  approaches  c  and  in  the  opposite  direction 
when  d  moves  away.  These  currents  go  through  the 
circuit  to  the  coil  c  at  B.  Their  effect  is  to  strengthen 
and  weaken  the  magnetic  field  acting  upon  d  (at  j5), 
attracting  and  repelling  d.  In  this  way  the  disk  at  B 
is  made  to  vibrate  just  like  that  at  A.  Its  vibrations 
cause  weak  sound  waves  which  may  be  heard  by  the 
ear  at  B. 

Note  that  the  sound  waves  are  produced  at  B ;  alter- 
nate currents  pass  through  the  wire  —  not  sound  waves. 


USES  OF  ELECTRICAL  ENERGY  165 

The  circuit  is  completed  by  allowing  the  current  to 
return  through  the  earth.  A  battery  is  commonly  used 
on  the  circuit  to  overcome  the  resistance  of  the  wire. 

189.  The  Telegraph.  —  The  sender  of  a  telegraph 
message  uses  a  key  k  (Fig.  121),  that  simply  closes  and 
opens  a  circuit.  In  the  distant  office  is  a  sounder  s,  by 
which  the  message  is  received.  Pressing  on  k  closes 
the  circuit;  the  current  then  magnetizes  the  electro- 
magnet m ;  this  draws  the  armature  a  so  that  it  strikes 


feu 


el 

FIG.  121 


the  frame  f,  making  a  clicking  sound.  The  key  being 
lifted,  m  is  demagnetized,  and  a  spring  pulls  a  till  it 
strikes  the  frame  above,  making  a  different  sound. 

If  these  two  sounds  are  separated  by  only  an  instant, 
a  dot  is  said  to  be  made;  when  a  longer  bit  of  time 
comes  between  them,  the  report  is  a  dash.  Every  letter 
is  represented  by  a  different  group  of  dots  and  dashes ; 
the  sender  can  make  each  at  will,  by  pressing  his  key 
for  a  shorter  or  longer  time.  Any  telegraph  operator 
must  learn  to  know  each  letter  instantly  by  the  sound 
of  the  dots  and  dashes  which  represent  it.  The  current, 
as  in  the  telephone,  goes  through  one  wire  and  returns 
to  the  battery  through  the  earth  (Fig.  121).  As  a  rule, 
the  batteries  are  composed  of  a  few  gravity  cells  placed 
at  intervals  along  the  circuit. 


166 


ELECTRICITY 


190.  Electric  Bells.  —  A  common  call  bell  is  shown 
in  Fig.  122.    The  hammer  or  striker  is  attached  to  a 
spring  s;  m  is  an  electro-magnet.    Follow  the  course  of 

the  current  carefully.  When 
the  circuit  is  closed,  m  is  mag- 
netized and  attracts  the  ham- 
mer; this  moving  quickly 
toward  m  strikes  the  bell  once. 
But  in  so  moving,  a  is  also 
moved  away  from  c,  breaking 
the  circuit  at  that  point.  At 
once  m  loses  its  magnetism 
and  the  spring  s  causes  the 
hammer  to  move  back  again; 
but  this  also  brings  a  again 
in  contact  with  c.  Thus  again 
the  circuit  is  closed,  m  is  mag- 
netized, the  hammer  hits  the 
bell,  and  all  is  repeated.  This  happens  very  rapidly, 
producing  the  familiar  buzzing  sound  of  electric  bells. 

191.  Electroplating,  —  Articles  covered  with  a  thin 
layer  of  metal  (gold,  nickel,  silver,  etc.)  are  said  to  be . 
plated.    Plating  is  commonly  done  by  use  of  the  electro- 
lytic effect  of  electrical  energy  (§  154).    The  articles  to 
be  plated  are  hung  in  water  that  contains  a  salt  (§  213) 
of  the  metal  to  be  used;  a  plate  of  the  metal  is  also 
hung  in  the  liquid.    This  plate   and  the   articles  are 
connected  by  separate  wires  with  a  dynamo  or  battery, 
in  such  a  way  that  the  current  has  to  pass  through 
the  liquid  (Fig.  123)  from  the  plate  to  the  articles.    The 


FIG. 122 


USES  OF  ELECTRICAL  ENERGY 


167 


current,  in  passing  through  the  water,  acts  upon  the  salt 
so  as  to  set  free  the  metal  that  it  contains.    This  metal, 


FIG.  123 

in  tiny  particles,  gathers  about  the  solid  bodies  through 
which  the  current  leaves  the  liquid,  thus  covering  those 
articles  with  a  metal  coating.  The  current  is  commonly 
furnished  by  a  dynamo,  though  a  battery 
of  cells  may  be  used  in  experiments. 

192.  Electric  Lights Electric  lights 

depend  upon  the  thermal  effect  of  elec- 
trical energy.  A  current  is  made  to  pass 
through  a  poor  conductor  against  great 
resistance  ;  in  doing  this,  it  heats  the  con- 
ductor until  it  is  luminous.  Two  sorts  of 
lamps  are  common,  —  incandescent  and 
arc  lamps. 

Fig.  124  shows  the  familiar  bulb  of 
an  incandescent  light.  The  fine  thread  of 
carbon  inside  the  bulb  offers  great  resist- 
ance to  the  current  that  is  sent  through  it. 
Thus  the  carbon  thread  becomes  very  hot  and  luminous. 
The  space  within  the  glass  bulb  is  a  nearly  perfect 


FIG.  124 


168 


ELECTRICITY 


vacuum;  if  air  were  admitted,  the  hot  carbon  would 
quickly  burn  up. 

In  the  arc  lamp  a  current  having  very  great  electro- 
motive force  is  made  to  pass  through  two  carbon  pencils 
placed  end  to  end.  Because  these  pencils 
just  loosely  touch  each  other,  great  resist- 
ance is  offered  to  the  current  at  that  point. 
The  current  flowing  against  this  resistance 
heats  the  ends  of  the  pencils  to  white  heat. 
Tiny  bits  of  the  carbon  are  detached  from 
one  pencil  and  pass,  in  a  glowing  condition, 
to  the  other  pencil.  Now  the  two  points  are 
drawn  apart  to  a  distance  of  about  a  quarter 
of  an  inch,  the  space  between  being  filled 
with  these  glowing  particles  (Fig.  125). 
This  space  filled  with  the  particles  is  called 
an  arc,  and  the  current  is  conducted  through 
it.  But  the  resistance  is  now  even  higher 
than  at  first;  thus  the  two  carbon  points  and  the  arc 
between  are  all  very  highly  heated,  so  that  they  glow 
and  give  a  bright  light. 

Arc  lamps  are  used  in  lighting  streets,  halls,  and 
large  rooms.    They  are  arranged  in  series  on  the  circuit 


FIG.  125 


IL_H_J 


FIG.  126 


(Fig.  126),  and  need  a  current  of  high  potential.  Each 
lamp  contains  a  device  for  keeping  the  carbon  points  the 
proper  distance  apart  as  they  are  consumed. 


USES  OF  ELECTRICAL  ENERGY  169 

193.  Wireless  Telegraphy.  —  Messages  are  now  trans- 
mitted without  wires  over  long  distances  by  means  of 
waves  in  the  ether.  These  waves  are  caused  when  elec- 
trical discharges  are  produced.  They  travel  with  great 
speed  in  all  directions  from  their  source,  growing 
weaker  as  they  advance.  Of  course  these  waves  can  do 
no  great  work  at  a  distance ;  but  delicate  instruments 
are  made,  by  means  of  which  the  weak  waves  serve  to 
close  the  circuit  of  a  local  battery  and  telegraph  receiver, 
so  that  this  receiver  shall  be  operated  by  them. 

QUESTIONS 

1.  Explain  the  action  of  the  electric  motor.    What  sort  of  cur- 
rent can  be  used  ?    Name  uses  of  electric  motors. 

2.  How  are  electric  cars  driven  ?    How  is  the  speed  of  the 
motor  and  car  controlled  ?    How  is  the  main  circuit  completed  ? 

3.  Explain  the  telephone.    What  travels  over  the  wires?  How 
are  the  sound  waves  that  the  listener  receives  produced  ? 

4.  How  is  a  telegraph  message  sent  by  an  operator?    Describe 
the  sounder  on  which  it  is  received.    Explain  the  action  of  the 
sounder. 

5.  Carefully  explain  the  common  electric  bell. 

6.  What  would  you  do  with  an  article  that  you  were  going  to 
plate  ?    What  is  put  into  the  water  ?    What  is  the  action  of  the 
current  upon  this  substance  ? 

7.  Of   what   electrical    effect    do   electric   lights   make   use? 
Explain  how  a  current  is  used  to  make  a  body  luminous.    What 
two  sorts  of  electric  lamps  are  common  ? 

8.  Explain  the  incandescent  light.    Why  is  the  carbon  thread 
placed  in  a  vacuum  ? 

9.  Explain  the  arc  light.    What  sort  of  a  current  is  needed, 
and  why  ?    From  what  are  the  light  waves  sent  out ;  that  is, 
what  parts  of  the  lamp  are  luminous? 


PAET  II.    CHEMISTEY 

CHAPTER   VIII 
OUTLINE   OF  CHEMICAL   STUDY 

SECTION   I 
GENERAL   INTRODUCTION 

194.  Chemistry.  —  Chemistry,  like  physics,  treats  of 
matter,  but  in  a  different  way.    Physics  is  the  science 
of   matter  with   regard  to    its    motions,   etc.,  whereas 
chemistry  is  the  study  of  substances  with  regard  to 
the  kind  of  matter  that  is  in  them.    For  example,  in 
physics  we  have  found  and  studied  several  forces  and 
their  action,  without  much  regard  to  the  sort  of  matter 
acted  upon,  while  in  chemistry  we  shall  be  constantly 
dealing  with  kinds  of  matter,  seeking  to  know  what  this 
or  that  substance  is  made  of,  how  it  may  be  made,  how 
it  may  be  destroyed,  what  can  be  made  from  it,  and 
the  like. 

195.  Chemical  Changes.  — Matter  may  of  course  be 
changed  in  many  ways.    The  size,  shape,  or  state  of  a 
body  may  be  altered;  it  may  be  hardened,  heated,  or 
crystallized,  etc.    All  such  changes,  which  do  not  affect 
the  substance  or  kind  of  matter  of  the  body,  are  called 
physical  changes.    But  when  any  substance  is  so  acted 

171 


172  OUTLINE  OF  CHEMICAL  STUDY 

upon  that  there  is  some  change  in  the  kind  of  matter,  a 
chemical  change  is  said  to  take  place. 

Experiment  107.  —  Dissolve  some  common  salt  in  water  until 
no  more  can  be  taken  up.  Has  any  change  occurred  ?  Now  boil 
the  salt  water  to  dry  ness.  Does  anything  remain  ?  Was  this  a 
physical  or  a  chemical  change  ? 

Experiment  108.  —  Place  a  bit  of  sulphur  in  an  old  spoon  and 
heat  it  over  an  alcohol  lamp  or  gas  burner.  The  sulphur  melts 
and,  if  still  heated,  vaporizes.  Hold  a  saucer  just  above  the 
spoon ;  sulphur  collects  on  the  saucer  in  tiny  particles.  What 
sort  of  change  ? 

Now  burn  a  bit  of  sulphur  in  the  spoon,  holding  the  saucer 
above  it  as  before.  No. sulphur  collects  on  the  dish.  In  burning, 
the  sulphur  unites  with  oxygen  from  the  air,  forming  a  different 
substance,  that  passes  off  as  a  gas.  Is  this  a  physical  or  a  chemi- 
cal change  ? 

196.  Composition  and  Decomposition.  —  In  a  general 
way,  any  chemical  change  falls  under  one  of  two  classes, 
—  composition  and  decomposition.     Composition  is  the 
process  of  uniting  two  or  more  substances  to  form  an- 
other.   When  a  substance  is  broken  up  into  the  two  or 
more  substances  that  compose  it,  the  process  is  called 
decomposition,  and  the  body  is  said  to  be  decomposed. 

197.  Kinds  of  Substances We  do  not  need  to  be 

told  that  there  is  an  almost  countless  number  of  differ- 
ent substances  on  earth.    Many  of  these  we  know  can 
be  made  from  simple   substances,  by  processes  which 
man  has  devised.    Others  are  found  in  the  rock  and 
soil  of  the  earth,  having  been  made  by  natural  processes 
long  ages  ago.    And  a  still  larger  number,  perhaps,  are 
made  by  the  growth  and  action  of  living  matter,  —  plant 


GENERAL  INTRODUCTION  173 

and  animal.  These  many  kinds  of  substances  may  be 
considered  in  three  different  classes,  —  elements,  com- 
pounds, and  mixtures. 

198.  Elements.  —  Of  this  great  number  of  substances 
there  will  of  course  be  some  that  are  composed  of  sev- 
eral simpler  ones.  These  simpler  ones  may,  in  turn,  be 
made  of  others  that  are  still  more  simple.  But  clearly 
we  cannot  go  on,  without  limit,  breaking  up  each  of 
these  simple  substances  into  simpler  ones;  that  is,  we 
must  soon  reach  substances  that  are  perfectly  simple  — 
that  cannot  be  broken  up  into  anything  else.  Such  sub- 
stances, that  cannot  be  divided  into  anything  else,  are  called 
elements.  They  are  absolutely  pure,  each  composed  of 
only  the  one  kind ;  the  smallest  particle  of  an  element 
would  be  of  just  the  same  nature  as  a  large  mass  of  it. 

Now  with  these  facts  clearly  in  mind,  we  shall  easily 
see  that  elements  cannot  be  made,  as  some  substances  are, 
by  composition,  since  each  is  composed  of  itself  only. 
The  elements  only  occur ;  that  is,  they  are  found  on 
earth,  sometimes  in  a  pure  state  but  more  often  united 
with  other  elements.  They  may  be  separated  from  these 
other  elements  by  different  methods,  called  analysis. 

Every  substance,  then,  is  made  of  elements ;  either  of 
one  alone  or  of  two  or  more  together.  Nearly  eighty 
elements  have  been  discovered  and  named.  Most  of 
these  are  uncommon.  Hardly  a  dozen  occur  in  very 
large  quantities.  Of  the  common  elements,  oxygen, 
hydrogen,  nitrogen,  and  chlorine  are  gases ;  mercury  is 
the  only  familiar  liquid ;  of  solids,  there  are  carbon, 
sulphur,  phosphorus,  and  some  metals  (§  212). 


174 


OUTLINE  OF  CHEMICAL  STUDY 


199.  Compounds.  —  When  two  or  more  elements  unite 
with  each   other  in  a  definite  proportion,  the   new  sub- 
stance formed  is  called  a  chemical  compound.    This  will 

be  explained  more  fully  later  (§§  202, 
203).  Note,  however,  that  the  ele- 
ments must  be  united,  that  is,  not 
simply  lying  side  by  side  in  the  same 
mass,  but  the  smallest  particles  of  each 
actually  combined  with  those  of  the 
others.  Also  note  that  the  result  is 
a  new  substance,  unlike  either  of  the 
elements  which  compose  it;  even  to 
its  molecules,  the  compound  is  differ- 
ent from  either  of  the  elements. 

Water  is  a  chemical  compound ;  its 
elements  are  hydrogen  and  oxygen  — 
both  gases.  Other  common  compounds  are  starch,  sugar, 
alcohol,  quartz,  and  many  acids,  bases,  and  salts. 

Experiment  109.  —  Put  a  small  piece  of  zinc  into  a  little  hydro- 
chloric acid  (an  inch  in  a  test  tube) ;  note  all  that  happens.  If 
the  zinc  does  not  finally  disappear,  add  more  acid.  When  the 
zinc  can  no  longer  be  seen,  boil  the  liquid  in  an  evaporating  dish 
(Fig.  127)  till  dry.  Examine  the  substance  that  remains.  Do 
you  think  this  a  compound  ?  Why  ? 

200.  Mixtures When  two  or  more  substances,  with- 
out uniting  chemically,  together  form  another  substance, 
that  mass  is  called  a  mixture.    A  mixture  may  differ  in 
some  ways  from  each  of  the  substances  that  compose  it, 
but  no  new  substance  is  formed ;  that  is,  the  mixture  has 
no  molecule  of  its  own,  being  composed  of  molecules  of 
each  substance  lying  side  by  side  but  not  combined. 


FIG.  127 


GENERAL  INTRODUCTION  175 

Mixtures  are  very  common ;  many  are  made  in  nature 
and  many  are  made  by  the  work  of  man.  Wood  and 
coal  are  mixtures,  also  the  air.  Various  sorts  of  vege- 
table and  animal  products  —  cloth,  paper,  leather,  and 
many  other  mixtures  —  are  common. 

The  difference^ between  chemical  compounds  and  mix- 
tures is  important.  In  compounds  the  elements  unite  to 
form  a  new  substance,  which  has  a  molecule  of  its  own ; 
that  is,  all  its  molecules  are  like  each  other  and  like  the 
mass.  In  a  mixture  the  molecules  are  those  of  the  ele- 
ments or  compounds  that  compose  it ;  thus  they  are  of 
different  kinds,  and  the  mixture,  as  a  substance,  has  no 
molecule  of  its  own.  For  further  explanation  see  §  207. 

Experiment  110.  —  Grind  small  quantities  of  sulphur  and  iron 
filings  together  in  a  mortar  till  well  mixed.  Draw  a  magnet 
through  the  mass.  Is  it  a  compound  or  a  mixture  ? 

QUESTIONS 

1.  Of  what,  in  general,  does  the  science  of  chemistry  treat? 

2.  Show  the  difference  between  physical  changes  and  chemical 
changes.    Give  examples  of  each.    Does  wood  suffer  a  physical 
or  a  chemical  change  when  burned? 

3.  What  is  meant  by  chemical  composition?    Define  decom- 
position.   Try  to  think  of  examples  of  each  process. 

4.  What  is  meant  by  an  element  ?    How  are  elements  gener- 
ally obtained  ?    How  many  have  been  discovered  ? 

5.  Are  masses  ever  formed  of  one  element  alone  ?    Name  some 
common  elements.    With  how  many  are  you  familiar? 

6.  What  is  a   chemical  compound?    How  does  a  compound 
mass  differ  from  an  elementary  mass  ?    Is  the  molecule  of  a  com- 
pound the  same  in  its  nature  as  the  mass  ? 

7.  What  is  a  mixture  ?  How  do  compounds  differ  from  mix- 
tures ?    Name  some  common  substances  that  are  mixtures. 


176  OUTLINE   OF   CHEMICAL   STUDY 

SECTION    II 
CHEMICAL   ACTION 

201.  The  Atomic  Theory.  —  The  name  molecule  has  been 
given  to  "  the  smallest  particle  of  any  substance  that 
can  exist  alone  "  (§  10).    Now  we  have  learned  that  ele- 
ments combine  to  form  compounds,  and  that  each  mole- 
cule of  a  compound  is  just  like  all  the  others.    This 
would  seem  to  show  that  each  molecule  must  contain 
in  itself  a  small  portion  of  each  of  the  elements  in  the 
compound.    But  this  at  once  raises  the  question,  How 
can  a  molecule  (the  smallest  particle  that  can  exist  alone) 
be  made  up  of  smaller  parts  ? 

Scientists  have  answered  the  question  by  formulating 
an  atomic  theory.  They  say  that  there  may  be  parti- 
cles smaller  than  molecules,  but  that  these  can  never 
exist  alone,  that  is,  they  must  always  be  united  with  at 
least  one  other.  These  smaller  particles  are  called  atoms. 
An  atom  may  unite  with  others  of  its  kind  or  of  dif- 
ferent kinds,  but  it  must  always  be  in  a  union. 

A  molecule,  then,  is  said  to  be  composed  of  atoms. 
Therefore  we  see  that  each  molecule  of  a  substance  may 
be  just  like  the  others,  and  yet  every  one  may  be  made 
up  of  atoms  of  different  elements.  In  other  words,  when 
two  or  more  elements  unite  to  form  a  compound,  the  mole- 
cules of  each  element  break  up  and  the  atoms  of  the  dif- 
ferent kinds  unite  with  each  other,  forming  molecules 
that  will  be  all  alike. 

202.  Chemical  Affinity.  —  The   atomic   theory  allows 
us  still  to  say  that  the  molecule  is  the  smallest  particle 


CHEMICAL  ACTION  177 

of  a  substance  that  can  exist  alone.  Clearly,  a  com- 
pound will  no  longer  exist  if  its  molecules  are  divided 
again,  for  each  molecule  is  made  of  atoms  of  different 
elements.  The  molecules  of  an  element  are  also  the 
smallest  bits  that  can  exist  alone,  each  molecule  being 
made  of  atoms  that  cannot  be  separated  unless  by  unit- 
ing with  other  different  atoms.  Between  elements  and 
compounds,  however,  there  is  this  difference :  the  atoms 
of  an  element  are  just  alike,  and  the  same  in  substance 
as  the  molecule,  whereas  the  molecule  of  a  compound 
is  made  of  different  kinds  of  atoms. 

In  any  molecule,  the  force  that  binds  the  atoms  together 
is  called  chemical  affinity.  Its  action  in  holding  atoms 
together  in  molecules  is  somewhat  similar  to  that  of 
cohesion,  which  binds  molecules  together  in  masses. 
Without  cohesion  we  should  have  no  masses  of  definite 
form  ;  without  chemical  affinity,  no  substances  of  definite 
composition. 

203.  Chemical  Combination.  —  When  two  or  more  ele- 
ments unite  to  form  a  compound  they  are  said  to  com- 
bine ;  the  process  is  called  chemical  combination.  The 
number  of  atoms  which  may  combine  to  form  a  mole- 
cule varies  according  to  the  substance  formed  ;  some 
molecules  contain  only  two  atoms,  while  others  contain 
nearly  one  hundred.  In  any  one  substance,  however, 
the  molecule  must  always  contain  the  same  number  of 
atoms;  moreover,  these  atoms  must  always  be  those  of 
the  same  elements,  and  each  element  must  always  be 
present  with  the  same  number  of  atoms.  For  example  : 
water  is  a  compound ;  a  molecule  of  water  must  always 


178  OUTLINE  OF  CHEMICAL  STUDY 

contain  three  atoms ;  two  of  these  atoms  must  be  those 
of  the  element  hydrogen,  and  one  atom  must  be  of 
oxygen.  If  the  two  elements  combined  in  any  other 
proportion  (say  one  atom  of  each),  or  if  they  combined 
with  any  other  element,  the  molecule  formed  would  not 
be  that  of  water.  Thus  we  may  say  that  chemical  com- 
bination takes  place  only  between  definite  proportions  of 
certain  elements. 

Every  element  does  not  by  any  means  combine  with 
every  other  element.  Some  elements  may  combine  with 
several  different  ones,  while  others  can  unite  directly 
with  only  two  or  three.  The  study  of  what  elements 
combine  with  certain  others  is  of  course  an  important 
part  of  the  chemist's  work. 

Sometimes  the  same  elements  may  combine  in  more 
than  one  proportion.  In  such  cases  the  resulting  com- 
pounds would  of  course  be  different.  For  example, 
hydrogen  and  oxygen  combine  to  form  water  (two  atoms 
of  hydrogen  and  one  of  oxygen),  while  if  two  atoms  of 
oxygen  unite  with  the  two  of  hydrogen,  a  very  different 
substance  is  formed.  The  three  elements,  carbon,  hydro- 
gen, and  oxygen,  combine  in  a  great  many  different 
proportions,  forming  as  many  different  compounds. 

204.  Decomposition.  —  When  a  compound  is  broken 
up  into  its  elements  it  is  said  to  be  decomposed. 

Naturally  each  element  has  a  stronger  affinity  for 
some  of  the  elements  with  which  it  may  combine,  than 
for  others.  Thus  in  some  compounds  the  elements  will  be 
more  strongly  united  than  in  others.  Some  compounds 
are  so  weak  that  they  slowly  decompose  if  simply  left 


CHEMICAL  ACTION  179 

to  stand  in  air  or  in  sunlight.  Such  compounds  are 
called  unstable.  Strong  compounds,  which  do  not  easily 
decompose,  are  said  to  be  stable.  Often  when  two  or 
more  compounds  are  mixed  together,  they  so  act  as  to 
decompose  each  other  ;  the  atoms  then  unite  with  others 
for  which  they  have  greater  affinity,  and  form  new 
substances. 

205.  Heat  assists  Chemical  Action.  —  Heat  is  a  very 
important  aid  to  chemical  action,  both  composition  and 
decomposition.  Many  changes  which  will  not  take 
place  at  ordinary  temperatures  easily  occur  if  the  sub- 
stances are  heated. 

Experiment  111.  —  Into  a  clean,  dry  test  tube  put  a  little 
sugar  ;  heat  gently.  Notice  what  occurs,  and  when  the  mass 
becomes  solid  examine  it.  Has  a  chem- 
ical change  occurred  ?  In  a  similar  way, 
treat  some  small  bits  of  wood  in  a  test 
tube,  and  examine.  Can  you  discover 
whether  heat  has  here  caused  composition 
or  decomposition  ? 

Experiment  112. — Put  a  small  piece 
of  lead  (a  BB  shot)  into  a  test  tube  and         .         FlG- 128 
add  a  little  cold  sulphuric  acid  (concentrated).    Look  for  any 
action.    Carefully  heat  the  acid  and  look  again  for  signs  of  action 
(Fig.  128).    In  this  case  the  change  includes  decomposition  and 
composition. 

Combustion,  or  burning,  is  a  very  common  sort  of 
chemical  action;  and  we  know  that  to  burn  any  com- 
mon substance  it  must  first  be  heated.  Gunpowder  and 
other  explosives  suffer  rapid  chemical  change  when 
heat  is  applied  ;  and  in  many  other  sorts  of  chemical 
action  we  find  that  heat  plays  an  important  part. 


180  OUTLINE  OF   CHEMICAL  STUDY 

206.  Heat  from  Chemical  Action.  —  Not  only  does  heat 
aid  chemical  action,  but  it  is  also  given  off  during  such 
activity.    Some  of  the  chemical  energy  set  free  during  a 
chemical  change  is  transformed  into  heat,  and  this  is  one 
of  our  important  sources  of  heat  (§  74). 

Experiment  113.  —  Put  a  small  piece  of  zinc  into  a  test  tube 
with  hydrochloric  acid.  Do  you  see  any  sign  of  chemical  action? 
Grasp  that  part  of  the  tube  where  the  acid  is.  What  further  evi- 
dence of  chemical  action  do  you  discover? 

Experiment  114.  —  Cut  a  bit  of  metallic  potassium  the  size  of 
a  small  pea.  Throw  it  upon  water  and  stand  away.  The  potas- 
sium decomposes  the  water,  setting-  free  its  hydrogen  and  oxygen. 
Is  this  a  chemical  action?  Do  you  note  any  sign  that  heat  is 
given  off  during  this  action  ? 

In  burning  a  substance  we  have  first  to  apply  heat 
to  it.  The  chemical  action  that  is  caused  by  this  gives 
off  enough  energy  itself  to  heat  more  of  the  mass ;  and 
so  the  combustion  keeps  on  by  its  own  heat,  until 
stopped  by  some  means. 

207.  Compounds  and  Mixtures.  —  We  have  seen  that 
in  a  compound  the  molecules  are  all  alike,  and  every 
one  contains  the  same  number  of  atoms  of  each  element 
in  the  substance.    That  is,  the  molecules  of  each  ele- 
ment have  been  broken  up,  their  atoms  combining  with 
other  atoms  to  form  a  new  sort  of  molecule.    In  a  mix- 
ture no  chemical  combination  takes  place.    The  molecules 
of  each  element  or  compound  lie  side  by  side  in  the 
mixed  mass ;  each  is  unchanged  and  no  new  molecule 
is  formed.    The  proportion  of  substances  in  a  mixture  is 
not  definitely  fixed,  as  in  a  compound,  but  the  same  ones 
may  be  mixed  in  any  proportion  whatever. 


CHEMICAL  ACTION  181 

Experiment  115.  —  Grind  a  mixture  of  one  half  ounce  of  iron 
filings  and  one  ounce  of  sulphur  in  a  mortar.  Examine  carefully 
and  draw  a  magnet  through  the  mass.  Is  it  a  compound  or  a 
mixture  still? 

Into  an  old  test  tube  put  a  little  of  the  mass,  and  heat  it  slowly 
but  well.  When  solid,  allow  the  mass  to  cool.  Break  the  tube 
and  examine  the  substance.  Is  it  iron?  Is  it  sulphur?  Is  it  a 
compound  or  a  mixture  ? 

208.  Symbols.  —  For  convenience,  a  system  has  been 
devised  so  that  names  of  elements  and  compounds,  and 
even  chemical  changes,  may  be  expressed  by  symbols. 

The  names  of  elements  are  generally  expressed  by 
their  first  letter,  or  two  letters :  hydrogen,  H  ;  oxygen, 
O ;  carbon,  C ;  calcium,  Ca ;  zinc,  Zn,  etc.  Moreover, 
the  symbol  for  any  element  (e.g.  C,  O,  or  H)  means  also 
one  atom  of  that  element.  To  express  more  than  one 
atom,  a  small  figure  is  placed  after  the  letter;  thus  H2 
means  "  two  atoms  of  hydrogen";  O3  means  "  three 
atoms  of  oxygen." 

The  symbol  for  a  compound  is  made  by  writing  the 
symbols  of  its  elements  in  order,  each  showing  the  num- 
ber of  its  atoms  in  the  substance.  Thus  HC1  means  that 
in  the  compound  hydrochloric  acid  one  atom  of  hydro- 
gen is  combined  with  one  of  chlorine ;  HNO3  (nitric 
acid)  is  a  compound  in  which  one  atom  of  hydrogen,  one 
of  nitrogen,  and  three  atoms  of  oxygen  are  combined. 

The  symbol  of  a  compound  (HC1,  HNO3,  etc.)  of  course  repre- 
sents one  molecule  of  the  substance.  Two  molecules  would  be  thus 
written,  2  HC1,  2  HNO3,  etc.;  three  molecules,  3  HC1,  etc.  The 
number  (2,  3,  etc.)  so  written  belongs  to  the  whole  group  of  ele- 
ments, and  means  that  in  the  whole  quantity  represented  the 
quantity  of  each  element  is  taken  just  that  number  of  times.  For 


182 


OUTLINE  OF  CHEMICAL  STUDY 


example,  2HNO3  means  "two  molecules  of  nitric  acid,  each 
containing  one  atom  of  H,  one  of  N,  and  three  of  O  " ;  and  it 
further  means  that  in  the  whole  quantity  (two  molecules)  there 
are  (2x1)  two  atoms  of  H,  (2  x  1)  two  atoms  of  N,  and  (2x3) 
six  atoms  of  O. 

A  list  of  the  more  common  elements  and  their  sym- 
bols follows.  In  cases  where  the  symbol  is  quite  unlike 
the  word,  it  has  generally  been  obtained  from  the  Latin 
name  of  the  element. 


Aluminium  .  .  Al. 

Antimony  .  .  .  Sb. 

Bismuth  ....  Bi. 

Boron B. 

Bromine  ....  Br. 

Calcium  ....  Ca. 

Carbon C. 

Chlorine  ....  Cl. 

Copper Cu. 

Fluorine  .         .  F. 


Gold Au. 

Hydrogen    .   .  .  H. 

Iodine I. 

Iron Fe. 

Lead Pb. 

Magnesium  .  .  Mg. 
Manganese  .  .  Mn. 
Mercury  ....  Hg. 

Nickel Ni. 

Nitrogen .  .  .  .  N. 


Oxygen    .  .  .  .  O. 
Phosphorus   .  .  P. 
Platinum    .  .  .  Pt. 
Potassium  .  .  .  K. 

Silicon Si. 

Silver Ag. 

Sodium    ....  Na. 
Sulphur    .  .  .  .  S. 

Tin     Sn. 

Zinc  .  .  Zn. 


QUESTIONS 

1.  Of   what   is   a   molecule   composed?    Can   these   particles 
exist  alone  ?   If  a  molecule  is  broken  up,  what  becomes  of  its 
particles  ? 

2.  In  what  way  do  the  molecules  of  an  element  differ  from  those 
of  a  compound  ?    What  is  chemical  affinity  ? 

3.  How  many  atoms  may  a  molecule  contain  ?    Can  two  mole- 
cules of  the  same  substance  contain  different  numbers  of  atoms? 
Can  the  atoms  in  them  be  arranged  in  different  proportions? 

4.  What  is  meant  by  chemical  combination  ?    When  elements 
combine,  can  each  then  be  seen  in  the  compound?    Can  any  ele- 
ment combine  with  every  other  element  ? 

5.  What  is  meant  by  decomposition  of  a  compound? 

6.  State  examples  to  show  that  heat  assists  chemical  action. 


CLASSES  OF  SUBSTANCES  183 

7.  Explain  how  heat  may  be  given  off  during  chemical  changes. 
Show  how  the  heat  set  free  during  combustion  serves  to  keep  up 
the  burning. 

8.  Explain  the  difference  between  a  compound  and  a  mixture. 

9.  Tell  everything  about  these  substances  that  you  can  learn 
from  their  symbols :  H2O  (water) ;  H2SO4  (sulphuric  acid)  ;  NaCl 
(common  salt)  ;  C12H22O11  (sugar) ;  C2H5OH  (alcohol). 

SECTION  III 
CLASSES  OF  SUBSTANCES 

209.  Acid-Forming  and  Base-Forming  Elements. — Two 
important  classes  of  substances  are  adds  and  bases. 
Some  elements  have  the  power  to  unite  with  others  to 
form  acids,  while  different  elements  in  a  similar  way 
usually  form  bases.  Elements  that  commonly  form  acids 
are  called  acid-forming,  or  negative,  elements;  those 
that  form  bases  are  called  base-forming,  or  positive, 
elements. 

The  common  negative  (acid-forming)  elements  are  bro- 
mine, carbon,  chlorine,  fluorine,  iodine,  nitrogen,  oxygen, 
phosphorus,  silicon,  and  sulphur.  Of  positive  (base-form- 
ing) elements  there  are  aluminium,  calcium,  copper,  gold, 
iron,  lead,  mercury,  nickel,  platinum,  potassium,  radium, 
silver,  sodium,  tin,  and  zinc.  The  element  hydrogen 
seems  to  hold  a  neutral  place,  being  found  in  both 
acids  and  bases. 

Sometimes  a  group  of  elements  acts  like  a  single  atom 
in  combining  with  other  elements ;  for  example,  NO8 
in  nitric  acid,  or  SO4  in  sulphuric  acid.  Such  a  group 
of  elements  is  called  a  radical.  Like  elements,  radicals 
are  either  positive  or  negative. 


184  OUTLINE  OF  CHEMICAL  STUDY 

210.  Acids.  —  An  acid  is  a  compound  made   up  of 
hydrogen  and  a  negative  element  or  radical.    Note  these 
symbols  of  acids :  HC1,  hydrochloric;   HBr,    hydrobro- 
mic;  HNO3,  nitric;  H2SO4,  sulphuric.    Acids  generally 
have  a  sharp  or  rather  sour  taste ;  they  often  act  upon 
other    compounds,    causing    chemical    changes;    some 
acids  act  strongly  upon  animal  matter,  and  some  are 
poisonous. 

The  sharp  taste  of  many  fruits  is  due  to  acids.  Lem- 
ons, raspberries,  and  currants  contain  citric  acid  ;  grapes 
contain  tartaric  acid;  apples  and  cherries,  malic  acid. 
Vinegar  owes  its  sour  taste  to  acetic  acid,  and  sour  milk 
contains  lactic  acid. 

211.  Bases.  —  A  base  is  composed  of  OH  in  combina- 
tion with  a  positive  element  or  radical.    OH  is  a  neg- 
ative radical ;  it  is  sometimes  called  hydroxyl. 

Four  bases  are  common:  NaOH,  sodium  hydrate; 
KOH,  potassium  hydrate;  Ca(OH)2,  calcium  hydrate; 
NH4OH,  ammonium  hydrate.  The  last  of  these,  NH4OH, 
is  diluted  with  water  and  used  for  household  purposes 
under  the  name  ammonia.  NaOH  and  KOH  are  used 
in  making  soap.  Ca(OH)2  is  sometimes  used  in  making 
other  bases. 

An  alkali  is  a  base  that  is  soluble  (can  be  dissolved) 
in  water.  The  strongly  basic  compounds  —  NH4OH, 
NaOH,  and  KOH  — are  alkalis. 

212.  Metals.  —  The    positive    or    base-forming    ele- 
ments are  commonly  called  metals.    We  usually  think 
of  a  metal  as  a  solid,  heavy,  and  rather  hard  substance. 
These  properties  are  true  of  some  metals,  but  not  of  all ; 


CLASSES  OF  SUBSTANCES  185 

for  example,  mercury  is  a  liquid ;  sodium  and  potassium 
float  upon  water  and  are  also  soft.  Thus  it  is  difficult 
to  find  any  common  property  by  which  to  define  a 
metal,  and  in  this  study  we  must  be  content  to  learn 
some  of  the  important  metallic  elements,  together  with 
their  general  behavior. 

By  far  the  greater  number  of  elements  are  metals. 
Some  of  these  are  very  common  on  earth,  while  others 
are  very  rare.  A  few  metals  (e.g.  iron,  copper,  and 
zinc)  are  of  much  importance  in  the  life  of  man;  but 
there  are  also  several  whose  existence  is  never  realized 
by  us,  and  whose  very  names  are  never  heard  except 
among  scientists. 

A  few  metals  are  sometimes  found  free  in  the  earth, 
though  most  of  them  occur  only  in  compounds  with 
other  elements.  Pure  metals  are  obtained  by  breaking 
up  the  salts  or  the  ores  in  which  they  occur.  The  fol- 
lowing metallic  elements  are  familiar:  Al,  Bi,  Ca,  Cu, 
Au,  Fe,  Pb,  Hg,  Ni,  Pt,  K,  Ag,  Na,  Sn,  Zn  (§  208).  Of 
these,  Ca,  Na,  and  K  are  not  common  in  a  free  (not 
combined)  condition. 

213.  Salts.  —  The  salts  form  a  large  and  important 
group  of  substances.  Many  different  salts  may  be 
formed  by  the  action  of  metals  upon  acids,  or  of  bases 
upon  acids.  In  either  case,  a  salt  is  formed  when  the 
hydrogen  of  an  acid  is  set  free  and  some  metal  taken  on 
in  its  place. 

Experiment  116.  —  To  a  little  hydrochloric  acid  (HC1)  in  a 
test  tube  add  a  piece  of  zinc  (Zn).  Note  the  action.  Bubbles 
show  that  a  gas  is  given  off  ;  this  is  hydrogen  (H).  When  the 
action  ceases,  boil  the  liquid  to  dryness.  Describe  the  substance 


186  OUTLINE  OF  CHEMICAL  STUDY 

that  is  left.  Is  it  zinc  ?  Is  it  hydrochloric  acid  ?  Is  it  a  new  sub- 
stance ?  Name  the  three  elements  that  you  had  in  the  test  tube 
at  first.  Which  one  of  these  escaped  ?  What  ones  have  united  ? 
The  substance  is  a  salt  —  zinc  chloride. 

Repeat  the  experiment,  using  HNO3  and  Hg ;  again,  using 
H2SO4  and  Cu.  What  elements  combine  in  each  case  ? 

Salts  are  named  usually  from  the  metals  and  the 
acids  that  compose  them.  For  example,  salts  of  H2SO4 
(sulphuric  acid)  are  called  sulphates:  Cu  and  H2SO4 
form  CuSO4,  copper  sulphate ;  Fe  and  H2SO4  form 
FeSO4,  iron  sulphate;  Zn  and  H2SO4  form  ZnSO4, 
zinc  sulphate ;  etc.  Salts  of  HNO3  (nitric  acid)  are 
called  nitrates:  NaNO3,  sodium  nitrate;  KNO3,  potas- 
sium nitrate ;  AgNO3,  silver  nitrate ;  etc.  Salts  of  HC1 
are  called  chlorides :  NaCl,  sodium  chloride  ;  KC1,  potas- 
sium chloride ;  CaCl2,  calcium  chloride ;  HgCl2,  mercuric 
chloride ;  etc.  Not  only  metals  but  positive  radicals  may 
unite  with  acids  to  form  salts.  The  positive  radical  NH4 
(ammonium)  forms  two  common  salts,  —  NH4C1,  ammo- 
nium chloride  (sal  ammoniac),  and  NH4NO3,  ammonium 
nitrate. 

Since  there  are  many  different  acids  and  metals,  the 
number  of  different  metallic  salts  is  great.  Some  of 
these  occur  in  the  earth  ;  NaCl  (common  salt)  is  very 
abundant,  also  KNO3  (saltpeter)  and  NaNO3.  Many 
salts  can  be  prepared  by  man,  and  in  some  cases  they 
are  prepared  by  him  in  great  quantities. 

The  uses  of  salts  are  also  numerous.  Some  are  useful 
as  foods,  notably  chlorides  and  phosphates ;  very  many 
are  used  in  medicine,  —  chlorides,  bromides,  phosphates, 
sulphates,  nitrates,  carbonates,  etc. ;  others  are  used  as 


CLASSES  OF  SUBSTANCES  187 

sources  from  which  to  obtain  acids  or  metals.  Salts  are 
used  in  many  mechanic  arts,  in  photography,  in  elec- 
troplating, electrotyping,  and  batteries;  in  plaster,  in 
fertilizers,  in  explosives,  and  in  other  ways. 

214.  Oxides.  —  Nearly  all  elements  combine  directly 
with  oxygen  (O)  ;  that  is,  each  forms  with  O  a  com- 
pound in  which  itself  and  oxygen  are  the   only  ele- 
ments.   The  compound  formed  by  the  direct  union  of 
an  element  with  oxygen  is  commonly  called  an  oxide. 

Some  oxides  are  solids  and  are  very  hard,  some  are 
gases,  and  still  others  are  liquids.  They  occur  very  often 
as  powders,  —  that  is,  masses  of  small  particles.  Iron 
rust  is  an  oxide  of  iron ;  lead  scraped  bright  and  then 
exposed  to  the  air  becomes  covered  with  a  thin,  dull 
coating  of  lead  oxide.  An  oxide  of  carbon,  CO2,  is  a 
gas;  it  is  found  mixed  with  the  air,  and  is  formed 
whenever  C  burns  in  O. 

Similarly,  sulphur  (S)  combines  directly  with  some 
elements  to  form  sulphides.  Of  these,  iron  sulphide 
(FeS2)  is  very  common ;  Cu,  Pb,  Sn,  and  Ag  also  form 
common  sulphides. 

215.  Minerals.  —  The  earth,  so  far  as  we  can  discover, 
is  composed  largely  of  rock  masses  (and  soil  on  the  sur- 
face) which  are  either  pure  minerals  or  mixtures  of  min- 
erals.   Mineral  substances  are  compounds,  —  commonly 
oxides,  carbonates,  or  sulphates.    The  oxide  of  silicon, 
SiO2,  is  very  common  —  we  call  it  quartz  ;  other  oxides 
are  those  of  Al,  Ca,  Mg,  K,  Na,  and  Fe.    The  important 
carbonate  is  that  of  calcium, CaCO3,  called  limestone;  and 
the  sulphate  of  calcium,  CaSO4  (gypsum),  is  also  common. 


188  OUTLINE  OF  CHEMICAL  STUDY 

216.  Ores.  —  Most  of  the  metals  are   found  in  the 
earth  in  the  form  of  ores.    An  ore  is  a  mineral  substance 
containing  a  metal  that  may  be  removed  from  it  for 
man's  use.    The  mineral  substance  may  be  any  sort  of 
rock  mass.    The  metal  itself  is  mixed  with  the  rock, 
sometimes  in  its  free  (or  uncombined)  state,  but  more 
often  as  an    oxide,  a  sulphide,  or  a  salt.    That  is,  if 
we  were  to  see  an  ore  of  some  metal,  we  should  see 
a   rock    in  which    were    scattered   masses    of   possibly 
the  pure  metal  itself,  but  more  likely  of  some  salt, 
oxide,  or  sulphide  of  the  metal.    Iron,  copper,  tin,  lead, 
silver,  gold,  zinc,  and  a  few  other  metals  are  taken 
from  ores. 

217.  Alloys.  —  An  alloy  is  a  mixture  of  two  or  more 
metals,  made  by  melting  them  together.    Many  alloys 
may  at  first  thought  seem  to  be  metals ;  they  are  not 
elements,  however,  but  are  made  by  man's  work.    Brass 
is  an  alloy  of  copper  and  zinc  ;  bronze  is  made  of  tin 
and  copper  ;   solder  contains  tin  and  lead ;  gun  metal 
and  bell  metal  contain  copper  and  tin  in  different  pro- 
portions.    G-erman  silver  is  an  alloy  of  copper,  zinc,  and 
nickel;  type  metal  contains  lead  and  antimony;  and  pew- 
ter is  an  alloy  of  lead  and  tin. 

218.  Hydrocarbons. — An  hydrocarbon  is  a  compound  of 
hydrogen  and  carbon.    There  are  many  hydrocarbons,  for 
these  elements  unite  in  various  ratios.  Two  common  hydro- 
carbons may  serve  as  examples :  acetylene  (C2H2),  a  com- 
mon illuminant  used  in  automobile  headlights ;  and  marsh 
gas  (CH4),  the  explosive  "  fire  damp  "  of  coal  mines.  Kero- 
sene and  other  petroleum  products  contain  hydrocarbons. 


CLASSES  OF  SUBSTANCES  189 

219.  Carbohydrates. — A  very  important  group  of  com- 
pounds can  be  made  from  the  elements  carbon,  hydrogen, 
and  oxygen.    They  are  made  in  nature,  chiefly  by  the 
activity  of  plants.    Because  of  their  composition  (G  and 
H2O)  they  are  called  carbohydrates.    Starch,  sugars,  and 
cellulose  are  common  carbohydrates  ;  they  occur  in  seeds, 
all  parts  of  living  plants,  and  fruits.    The  carbohydrates 
form  a  very  important  part  of  the  food  of  most  animals. 

220.  Proteids.  —  Another  group   of  substances  that 
are  necessary  to  the  life  of  higher  animals  is   called 
proteids.    These  contain  the  elements  carbon,  hydrogen, 
oxygen,  and  nitrogen;  sometimes  sulphur  or  phosphorus 
also.    Proteids  occur  in  the  white  of  eggs,  in  lean  meat, 
cheese,  wheat  flour  (gluten),  gelatin,  etc. 

221.  Solutions.  —  When  a  substance  is  dissolved  in  a 
liquid  it  is  said  to  be  in  solution.    The  liquid  in  which 
a  substance  is   dissolved  is   called   a   solvent.    Certain 
solids,  liquids,  or  gases  may  be  thus  put  in  solution; 
their  molecules  are  separated  and  they  mix  with  those 
of  the  solvent.    This  mixture  of  a  substance  in  a  solvent 
is  called  a  solution.    There  is,  of  course,  a  limit  to  the 
amount  of  any  given  substance  that  a  liquid  can  dis- 
solve.   When  the  solvent  holds  in  solution  all  that  it 
can  dissolve  of  any  substance,  the  solution  is  said  to  be 
saturated.    In  the  case  of  solids,  dissolving  is  hastened 
if  the  solvent  be  heated.    It  is  well  known  that  some 
substances  dissolve  better  in  hot  water  than  in  cold. 
Stirring  or  shaking  assists  solution  by  mixing  the  parti- 
cles more  rapidly.    A  substance  that  can  be  dissolved 
in  a  liquid  is  said  to  be  soluble. 


190  OUTLINE  OF  CHEMICAL   STUDY 

Experiment  117.  —  Dissolve  the  following  substance^  in  equal 
volumes  of  water  :  common  salt,  sugar,  sal  ammoniac,  ammonium 
nitrate,  magnesium  sulphate,  and  calcium  sulphate.  Note  how 
much  of  each  can  be  dissolved  in  the  water.  Which  are  the  more 
soluble  ? 

Which  of  these  substances  are  soluble  in  water:  HC1,  oil, 
alcohol,  kerosene,  molasses,  mercury,  and  NH4OH  ? 

Experiment  118.  —  Into  two  equal  volumes  of  water  put  equal 
quantities  of  sugar.  Stir  one  and  allow  the  other  to  stand  quietly. 
Which  dissolves  more  rapidly  ? 

Again  try  to  dissolve  two  equal  quantities  of  sugar  in  equal 
volumes  of  water,  one  cold  and  the  other  heated.  Try  to  dissolve 

some  lead  chloride  (PbCl2)  in 
cold  water  in  a  test  tube ;  now 
heat  the  water  (Fig.  129)  and 
note  the  result.  Does  heating 
help  in  dissolving  solids  ? 


Some  substances  are 
much  more  soluble  than 
others,  when  put  into  the 
FIG.  129        ^/|f/          same    liquid  ;   and   many 
that  will  not  mix  with  one 

solvent  will  dissolve  in  another.  Of  the  solvents,  water 
is  the  most  common  and  important.  Many  salts,  acids, 
and  bases  are  soluble  in  it,  besides  some  other  substances. 
For  this  reason  water  is  widely  used  as  a  cleansing  agent. 
Alcohol  is  also  a  common  solvent ;  tinctures  and  essences 
are  solutions  of  different  things  in  alcohol,  and  its  use 
in  medicines  is  important.  Many  of  the  fats  and  oils 
are  soluble  in  the  alkalis,  such  as  NH4OH,  NaOH,  and 
KOH.  Mercury  dissolves  several  of  the  metals,  forming 
amalgams.  Ether,  turpentine,  and  carbon  disulphide  are 
also  used  as  solvents  for  certain  substances. 


CLASSES  OF  SUBSTANCES  191 

QUESTIONS 

1.  What  class  of  elements  are  commonly  called  negative? 
What  are  positive  elements  ?    Name  some  elements  in  each  class. 
What  is  a  radical  ?    Name  two  radicals. 

2.  Define  an  acid.    What  properties  do  acids  generally  pos- 
sess?   Name  any  acids  that  you  can.    Name  any  substances  that 
you  think  may  contain  an  acid. 

3.  What  is  a  base  ?   Name  four  common  bases.    For  what  are 
these  sometimes  used  ?•   What  is  an  alkali  ? 

4.  Name  several  metals.    Are  they  elements,  compounds,  or 
mixtures  ?    How  are  the  metals  generally  obtained  ? 

5.  Under  what  conditions  is  a  salt  formed?    Name  any  salts 
that  you  can.    Are  salts  very  numerous  ?    Are  they  important  ? 

6.  In  what  ways  are  salts  obtained  ?    Name  uses  of  salts. 

7.  What  is  an  oxide?    Name  any  oxides  that  you  know  of. 

8.  What  sort  of  substances  compose  minerals?   Are  minerals 
compounds  or  mixtures  ? 

9.  What  is  an  ore  ?    Does  the  metal  in  an  ore  occur  in  a  free 
state  or  combined  with  other  elements?    Name  metals  that  are 
obtained  from  ores. 

10.  What  is  an  alloy?    Name  some  common  alloys. 

11.  Name  the  elements  that  compose  hydrocarbons.    Name 
any  common  hydrocarbons. 

12.  What  elements  combine  to  form  carbohydrates?    Name 
any  such  compounds,  telling  where  they  commonly  occur. 

13.  What  is  a   solution?    Is  it  a  confound  or  a  mixture? 
Name  some  common  solvents.    What  is  a  soluble  substance  ? 

14.  When  is  a  solution  saturated  ?    What  condition  in  the  sol- 
vent may  assist  the  dissolving  ?    Of  what  use  are  proteids  ? 

15.  What  sort  of  substance  is  a  tincture  ?    What  is  an  essence  ? 
What  is  an  amalgam  ? 

16.  Name  the  class  of  substances  to  which  each  of  the  follow- 
ing belongs  :  iron  ;  copper  sulphate  ;  sugar ;  mercury ;  kerosene  ; 
common  salt ;  gelatin ;  household  ammonia ;  starch  ;  iron  rust ; 
lead;  brass;  gasoline. 


CHAPTER    IX 


COMMON  SUBSTANCES 

SECTION  I 
ELEMENTS 

222.  Oxygen.  —  Oxygen  is  a  gas  without  color  or 
odor ;  it  occurs  most  widely  of  all  the  elements.  Many 
salts  and  acids,  and  all  bases,  carbohydrates,  and  oxides 
contain  O.  It  is  also  a  very  important  element  in  water, 

air,  and  the  solid 
earth.  In  the  air 
oxygen  is  free  (not 
combined  with 
other  elements), 
and  it  serves  two 
great  purposes  — 
it  supports  combus- 
tion (burning)  and 
helps  to  support 
animal  life.  We 

see  its  importance 

FIG  130  i 

at  once  when  we 

know  that  without   O  in  the    air  animals   could  not 
live  and  common  fires  would  not  burn. 

Experiment  119.  —  Fit  a  stopper  to  a  large  test  tube.  Perfo- 
rate the  stopper  with  a  round  file  and  push  through  this  hole  one 
end  of  a  glass  tube,  bent  as  in  Fig.  130.  Hang  the  whole  on  a 

192 


ELEMENTS 


193 


ring  stand,  so  that  the  other  end  of  the  tube  shall  dip  below  the 
surface  of  water  in  a  large  vessel  (see  Fig.  130).  Into  the  tube 
put  5  grams  of  potassium  chlorate  (KC1O3)  mixed  with  5  grams 
of  manganese  dioxide  (MnO2).  Stop  the  tube  tightly  and  heat  it. 
Bubbles  of  gas  soon  appear  in  the  water. 
Now  fill  two  or  three  pint  jars  with  water; 
tip  one  bottom  upward  under  water  and 
hold  it  over  the  tube  so  that  the  gas  shall 
go  up  into  it.  Be  sure  that  the  jar  is  full 
of  water  at  the  start,  and  allow  no  air  to 
enter  it.  As  the  gas  flows,  the  water  in  the 
jar  is  pushed  down  and  out.  When  the  jar 
is  full  of  gas  (i.e.  the  water  is  all  out  of  it), 
cover  it  with  a  piece  of  stiff  cardboard  or 
glass  and  lift  it  from  the  water.  In  the  same  way  fill  two  other 
jars,  and  keep  each  covered  (Fig.  131).  The  gas  is  O.  By  heat- 
ing, KC1O3  is  decomposed  into  KC1  and  3  O. 

Experiment  120.  —  Into  one  jar  of  O  put  a  glowing  splinter  of 
wood.  In  another  hold  a  bit  of  burning  sulphur  (Fig.  132).  In 
the  third  place  a  lighted  bit  of  candle.  Be 
careful  to  keep  the  jars  covered  as  much  of  the 
time  as  possible.  In  each  case  what  do  you 
notice  when  the  burning  substance  is  first  put 
into  the  jar  ?  What  do  you  notice  after  it  has 
burned  a  few  moments  ?  Try  to  explain  this. 


FIG.  131 


FIG.  132 


These  substances  seem  to  burn  better 
in  the  jar  of  pure  O  than  in  the  air.  In 
either  case  the  burning  is  a  process  of 
chemical  combination,  the  substance  com- 
bining with  oxygen  ;  in  the  jar  the  O  is 
nearly  pure,  while  in  the  air  it  is  mixed  with  a  much 
larger  quantity  of  another  gas  (N),  so  that  the  bodies 
burn  better  in  the  jar.  Note  carefully  that  when  sub- 
stances burn  in  air  (e.g.  wood,  kerosene,  paper,  etc.),  it 


194  COMMON  SUBSTANCES 

is  because  some  of  their  elements  are  uniting  with  the 
O  of  the  air.  It  is  for  this  reason  that  a  draught  of 
air  is  necessary  in  stoves,  lamps,  and  various  fires. 

223.  Hydrogen.  —  Hydrogen  also  is  a  colorless  and 
odorless  gas.  It  is  an  important  element,  forming  a 
part  of  all  acids  and  of  water.  Animal  and  vegetable 
substances  contain  large  quantities  of  H,  but  it  is  not 
common  in  its  free  state.  The  element  may  be  sepa- 
rated from  acids  by  the  action  of  metals  upon  them 
(§  213).  Hydrogen  is  the  lightest  substance  known, 
being  14 J-  times  lighter  than  air.  Balloons  are  usually 
filled  with  it,  so  as  to  make  them  rise  in  the  air. 

Experiment  121.  —  Arrange  the  apparatus  the  same  as  for  mak- 
ing O  (Experiment  119),  filling  jars  w.th  water.  Into  the  test  tube 
put  5  grams  of  zinc  with  5  cubic  centimeters  (cc.)  each  of  water 
and  HC1,  but  do  not  heat.  After  the  gas  has  flowed  a  few  seconds, 
collect  some  in  jars  by  the  same  method  as  in  Experiment  119. 
This  gas  is  H.  2  HC1  +  Zn  =  ZnCl2  +  2  H. 

Experiment  122.  —  Uncover  one  jar  of  H,  at  once  holding  a 
lighted  match  near  the  opening.  Always  be  careful  with  H,  for  it 
burns  in  air  and  explodes  if  mixed  with  it.  Using  a  small  jar, 
partly  uncover  it  for  an  instant;  then  cover  it  again  and  shake  it 
once.  Now  apply  a  match  to  the  opened  jar,  being  careful  not  to 
get  the  face  too  near.  If  the  air  and  H  are  mixed,  a  slight  explo- 
sion may  occur.  Does  H  burn  in  the  air?  Does  it  kindle  easily? 
With  what  element  does  it  combine  ? 

Hydrogen  combines  with  0  very  easily  if  it  be  heated 
to  its  kindling  temperature;  once  lighted,  it  burns 
readily.  Pure  H  burning  in  pure  O  makes  a  very  hot 
flame.  It  is  the  H  in  many  substances,  such  as  kero- 
sene, paraffin  (candles),  wood,  paper,  etc.,  that  makes 
them  kindle  easily. 


ELEMENTS  195 

224.  Nitrogen. — Like  H  and  O,  nitrogen  is  a  colorless 
and  odorless  gas.    It  occurs  free  in  the  air,  nearly  four 
fifths  of  the  air  being  N.    In  combination  with  O  (i.e.  NO3) 
it  forms  a  part  of  those  salts  that  are  called  nitrates,  and 
it  is  a  factor  in  the  proteids,  which  occur  mostly  in  ani- 
mal matter.    N  is  not  an  active  element,  and  it  does  not 
support  combustion.    Owing  to  this  last  fact,  N  in  the 
air  serves  a  very  great  use  by  checking  fires;  that  is, 
if  a  larger  portion  of  the  air  were  O,  fires  would  burn 
more  fiercely  and  they  could  not  be  controlled  so  easily. 

225.  Carbon. — The  element  carbon  is  a  solid.    Several 
substances  are  nearly  pure  C ;  for  example,  charcoal, 
coke,  lampblack,  boneblack,  and  gas  carbon.    Coal  also 
contains  a  large  amount  of  carbon.    Notice  that  each 
of  these  substances  is  one  that  remains  after  some  com- 
pound has  been  broken  up;  for  example,  charcoal  is 
left  when  wood  is  burned  imperfectly,  lampblack  when 
oils  are  burned  without  a  good  supply  of  air,  etc.    This 
shows  that  C  occurs  in  compounds  which  may  be  broken 
up  by  heat.    The  gases  in  the  compounds  are  first  driven 
off,  leaving  the  C.    If  plenty  of  air  be  supplied  and  the 
heat  be  great  enough,  C  will  combine  with  O  (i.e.  will 
burn)  and  pass  off  as  a  gas,  CO2  (carbon  dioxide). 

Experiment  123.  —  Burn  a  match  (wooden)  in  air,  allowing  it 
to  burn  completely.  How  much  ash  remains  ?  Now  break  up  the 
wood  of  a  match  or  a  splinter  into  bits,  place  these  in  a  test  tube, 
cover  with  a  little  dry  sand,  and  heat  over  a  flame.  Do  you  see 
any  evidence  of  decomposition?  What  remains  in  the  tube? 
Explain  the  difference  between  this  result  and  that  from  the 
burning  in  air.  Similarly,  heat  some  sugar  in  a  test  tube  till  it  is 
solid.  Note  and  explain  the  result. 


196  COMMON  SUBSTANCES 

Carbon  occurs  very  commonly  in  living  matter,  partic- 
ularly in  vegetable  substances.  In  these  cases  it  is 
nearly  always  combined  with  other  elements,  usually  O 
and  H.  The  element  occurs  free  in  two  forms,  diamond 
and  graphite.  Diamond  is  the  hardest  of  minerals,  and 
graphite  one  of  the  softest ;  both  are  crystalline,  and  each 
is  nearly  pure  C.  Graphite  mixed  with  clay  is  used  as 
"  black  lead  "  in  pencils. 

226.  Sulphur.  —  Sulphur  is  a  solid  element,  brittle, 
and  of  a  yellow  color.    It  occurs  free   in  the  earth, 
especially  near  volcanoes ;  it  also  occurs  combined  with 
metals   in   sulphides    and   sulphates.    It   burns   easily, 
forming  with  O  a  gas,  sulphur  dioxide  (SO2).    The  com- 
pounds of  sulphur  (e.g.  FeS2,  H2SO4,  H2S,  etc.)  are  of 
great  importance  to  man.    In  its  free  state,  S  is  used  in 
preparing  matches,  gunpowder,  and  rubber  goods ;  also 
in  medicine.    Sulphuric  acid  (H2SO4)  is  one  of  the  most 
important  of  chemical  compounds. 

227.  Phosphorus.  —  Phosphorus  is   a  solid   element, 
slightly  yellow  in  color,  and  of  a  waxy  nature  at  usual 
temperatures.    It  is  an  acid-forming  element  and  occurs 
largely  in  phosphates.    P  is  very  active,  combining  with 
several  elements  directly  and  at  low  degrees  of  heat. 
It  should  always  be  kept  and  cut  under  water. 

Experiment  124.  —  Cut  a  piece  of  P  no  larger  than  half  a  small 
pea.  Dry  this  on  blotting  paper  and  place  it  in  an  evaporating 
dish.  Place  a  bit  of  iodine  so  as  to  touch  the  P.  Do  you  notice 
anything  that  is  unusual  ? 

Caution.  Do  not  touch  P  with  the  hands,  and  do  not  breathe 
the  fumes  from  burning  P.  The  substance  is  very  poisonous. 
Also  be  careful  never  to  leave  the  least  bit  lying  around. 


ELEMENTS 


197 


Combination  of  P  with  O  also  takes  place  very  easily. 
Sometimes  P  will  burn  as  soon  as  it  is  placed  in  the 
air,  especially  if  it  be  cut  or  rubbed  a  little.  Owing  to 
the  ease  with  which  it  kindles,  P  is  commonly  used  in 
making  matches.  The  red  tip  contains  some  P  mixed 
with  other  substances.  Simple  rubbing  heats  this  tip 
enough  to  make  the  P  burn,  and  this  kindles  the  wood. 
P  gives  out  a  faint  glow  in  the  dark;  hence  it  is  used 
in  luminous  paint,  etc. 

228.  Chlorine.  —  Chlorine  is  a  greenish-yellow  gas  hav- 
ing a  disagreeable  odor.  It  is  not  common  in  a  free  state, 


but  occurs  in  a  group  of  salts  called  chlorides.  With 
hydrogen  Cl  forms  hydrochloric  acid,  HC1.  The  pure  ele- 
ment acts  strongly  upon  the  throat  and  lungs  if  inhaled. 
It  is  used  as  a  disinfectant  and  as  a  bleaching  agent. 

Experiment  125.  —  Arrange  apparatus  as  in  Fig.  133,  passing 
the  tube  to  the  bottom  of  a  jar  through  a  loosely  fitting  cover  of 
cardboard.  Into  the  test  tube  put  5  g.  of  MnO2  and  10  cc.  of 


198  COMMON  SUBSTANCES 

HC1.  Heat  the  mixture.  Cl  gas  is  set  free  and  flows  into  the  jar, 
driving  out  the  lighter  air.  Do  not  breathe  any  of  this  gas.  Note 
the  color  of  Cl.  This  is  one  of  the  few  gases  that  have  color  and 
can  be  seen.  (If  Cl  is  accidentally  inhaled,  pour  alcohol  on  a 
cloth  and  breathe  through  the  cloth  for  a  few  moments.) 

Experiment  126.  —  When  the  gas  in  the  jar  is  very  yellow, 
remove  the  flame,  wait  a  half  minute,  then  remove  the  glass  tube 
from  the  jar,  keeping  the  jar  covered.  Now  moisten  a  small  piece  of 
colored  calico,  drop  it  into  the  Cl,  and  quickly  cover  the  jar  again. 
If  no  change  is  noticed  soon,  try  another  piece  of  a  different  color. 

A  substance  called  bleaching  powder  is  much  used  in  bleaching 
cloth  and  paper,  because  it  contains  Cl. 

229.  Iron.  —  Iron  is  the  most  important  of  metallic 
elements  in  man's  work.  Its  uses  are  too  common  to 
need  mention  here.  The  element  occurs  in  several  ores, 
usually  combined  with  O  or  S.  The  sulphide,  FeS2, 
is  commonly  called  pyrite.  Iron  is  obtained  from  its 
ores  by  heating  them  in  a  blast  furnace.  In  this  big 
furnace  coke  or  coal  is  mixed  with  the  ore  (usually  an 
oxide  of  iron)  and  burned.  A  blast  of  air  is  forced  into 
the  furnace,  and  the  fire  (which  burns  all  the  time)  gives 
a  very  great  degree  of  heat.  In  this  heat  the  ore  is 
decomposed;  its  O  unites  with  the  C  of  the  coke,  and 
the  iron  in  a  melted  state  collects  at  the  bottom  of  the 
furnace.  From  here  it  is  drawn  off  into  molds,  and  is 
called  pig  iron  or  cast  iron.  It  is  very  impure. 

Steel  is  a  better  grade  of  iron,  which  contains  a  fixed 
amount  of  carbon.  It  is  commonly  made  by  blowing  air 
through  a  mass  of  highly  heated  pig  iron.  The  impuri- 
ties in  the  iron  unite  with  the  O  of  the  air  and  are 
thus  burned  off,  and  then  a  known  amount  of  carbon 
is  mixed  with  the  heated  mass. 


ELEMENTS  199 

230.  Sodium  and  Potassium.  —  The  solid  metallic  ele- 
ments Na  (sodium)  and  K  (potassium)  are  not  found  free 
in  nature.    Their  salts,  however,  are  very  common  and 
important.    The  elements  may  be  separated  from  some 
of  their  salts.    Neither   is  common  outside  of  labora- 
tories, and  no  great  use  is  made  of  them.    They  are 
soft,  waxy  metals,  lighter  than  water.    Na  acts  upon 
water  to  decompose  it,  and  K  does  the  same,  but  more 
strongly. 

Experiment  127.  —  Cut  a  small  piece  each  of  Na  and  K  (the 
size  of  a  small  pea).  Throw  the  Na  on  some  water  in  a  dish, 
being  careful  then  to  keep  away  from  it.  Next  do  the  same  with 
the  K.  What  difference  in  these  two  cases  ?  Try  to  explain  how 
this  difference  proves  that  K  acts  upon  water  more  strongly  than 
Na  (see  §  206). 

Of  the  salts  of  Na,  NaCl  is  common  and  important ; 
also  Na2SO4  (sodium  sulphate)  and  NaNO3.  Potassium 
carbonate,  K2CO3,  occurs  in  the  earth,  is  absorbed  by 
plants,  and  forms  a  part  of  wood  ashes ;  KNO3  (salt- 
peter) is  an  important  salt  of  K.  Na  and  K  form  strong 
bases  or  alkalis,  —  NaOH  and  KOH. 

231.  Calcium.  —  Calcium  (Ca)  is  a  solid  metallic  ele- 
ment ;  like  Na  and  K,  it  is  common  only  in  compounds 
with  other  elements.    Some  of  its  compounds,  however, 
are  important  and  are  found  in  large  quantities.    CaCO3, 
calcium  carbonate,  occurs  widely  in  the  earth  ;  in  differ- 
ent forms  it  is  called  limestone,  marble,  or  chalk.    CaSO4, 
sometimes  called  gypsum,  also  occurs  in  the  earth;  when 
heated   it  forms  a  white   powder,  —  plaster  of   Paris. 
The  oxide  of  calcium,  CaO,  is  called  lime;  it  is  used 


200  COMMON  SUBSTANCES 

in   making   mortar  and   plaster.    Ca  is  a  base-forming 
element,  the  base  being  calcium  hydrate,  Ca(OH)2. 

232.  Mercury. — The  element  mercury  (Hg)  is  a  metal, 
which  is  a  liquid  at  ordinary  temperatures.  It  is  some- 
times called  quicksilver.  It  occurs  free  in  rocks,  and 
also  as  the  sulphide  (HgS).  The  metallic  Hg  is  a  heavy 
liquid,  13.6  times  heavier  than  water.  It  is  used  in  ther- 
mometers and  barometers  ;  also  in  forming  the  reflecting 
surface  of  mirrors.  It  dissolves  several  metals,  and  for 
this  reason  it  is  used  in  separating  silver  and  gold  from 
their  ores. 

Hg  forms  several  salts,  among  them  two  chlorides, 
—  HgCl  and  HgCl2.  HgCl,  mercurous  chloride,  is 
called  calomel  and  is  used  in  medicine.  Mercuric  chlo- 
ride, HgCl2,  is  called  corrosive  sublimate;  it  is  used  as  a 
disinfectant  to  kill  bacteria.  Hg  is  a  poison,  as  are  some 
of  its  compounds. 


233.  Other  Metallic  Elements.  —  The  metals  copper 
(Cu),  lead  (Pb),  tin  (Sn),  zinc  (Zn),  silver  (Ag),  and  gold 
(Au)  are  familiarly  known  to  us.  Cu,  Ag,  and  Au  occur 
free  in  nature ;  Cu  and  Sn  occur  as  oxides ;  Cu,  Pb, 
Zn,  and  Ag  occur  as  sulphides,  and  Zn  as  a  carbonate. 
These  metals  are  in  common  use,  and  we  can  easily  think 
of  the  uses  of  each.  Cu  and  Zn  form  sulphates  that  are 
common,  and  Pb  forms  lead  carbonate,  ^PbCO3,  known 
as  white  lead  and  used  in  making  paint.  Silver  easily 
forms  the  sulphide  Ag2S ;  as  there  is  nearly  always  a 
little  S  in  the  air,  and  always  some  given  off  in  the  per- 
spiration from  the  body,  silver  articles  become  coated 
with  Ag2S  and  are  said  to  "  tarnish." 


ELEMENTS  201 

The  metal  aluminium  (Al)  is  a  very  important  ele- 
ment; it  occurs  as  a  part  of  some  of  the  most  abundant 
kinds  of  rocks.  As  a  metal,  Al  is  silver-white,  strong, 
but  very  light  in  weight.  The  oxide  of  Al,  A12O3, 
occurs  in  nature  as  sapphires  and  rubies ;  it  is  also 
powdered  and  used  for  polishing,  being  called  emery. 
Common  alum  is  a  sulphate  of  Al  and  K,  A1K(SO4)2. 

234.  Silicon.  —  The  element  silicon  (Si)  never  occurs 
free  in  nature,  though  in  compounds  it  is  very  abundant 
in  the  earth.  The  most  common  compound,  SiO2,  is 
called  silica  or  quartz ;  it  is  a  white  or  colorless  rock, 
and  in  a  finely  broken  form  it  is  white  sand.  Silicates 
are  salts  of  silicic  acid,  H2SiO3. 

QUESTIONS 

1.  What  sort  of  a  substance  is  O?    In  what  does  it  occur? 
How  common  is  O  ?    What  two  important  uses  does  it  serve  ? 

2.  Describe  the  element  H.    Why  is  H  used  in  balloons  ?   What 
chemical  property  makes  it  a  very  important  element  ?    In  what 
substances  does  it  occur? 

3.  In  what  does  N  occur?    Is  it  an  active  element?    What 
purpose  does  it  serve  in  the  air? 

4.  Name  substances  that  are  nearly  pure  carbon.    How  are 
these  substances  made  ?    In  what  sorts  of  matter  is  C  very  impor- 
tant?   Name  two  different  crystalline  forms  of  C. 

5.  Describe  the  element  S.    Where  does  it  occur  in  nature, 
and  in  what  form  ?    What  important  compounds  contain  S  ?    For 
what  is  free  S  used? 

6.  For  what  chemical  property  is  P  a  valuable  element?    Ex- 
plain its  use  in  matches.    Why  cut  P  under  water?  Why  not 
breathe  its  fumes? 

7.  Describe  chlorine.  What  important  compounds  does  it  form  ? 
What  uses  are  made  of  the  element  ? 


202  COMMON  SUBSTANCES 

8.  In  what  form  does  iron  occur  in  the  earth  ?    What  is  the 
symbol  for  iron  ?    How  is  iron  obtained  from  its  ores  ?    What  is 
pig  iron  ?    How  is  steel  made  ? 

9.  Name  salts  of  Na  that  are  important ;  also  salts  of  K. 
Explain  the  action  of  these  elements  upon  water. 

10.  Name  important  salts  of  Ca  that  occur  in  the  earth,  and  give 
common  names  for  each.    What  is  lime  ?    For  what  is  lime  used  ? 

11.  Describe  the  element  Hg.    Name  some  of  its  uses.    Name 
two  of  its  common  salts,  stating  the  use  of  each. 

12.  In  what  form  does  each  of  these  metals  occur :  Cu,  Ag, 
Au,  Pb,  Sn,  Zn  ?    Name  uses  of  each. 

13.  Describe  the  element  Al.    In  what  forms  does  it  occur? 
What  familiar  compounds  does  it  form? 

14.  In  what  form  does  Si  largely  occur?    What  is  its  most 
common  compound?    What  is  a  silicate? 

SECTION  II 
COMPOUNDS 

235.  Water.  —  Water  is  a  compound  of  hydrogen  and 
oxygen;  its  molecule  contains  two  atoms  of  H  and  one 
atom  of  O,  its  symbol  therefore  being  H2O.  Water  is  a 
most  important  compound,  occurring  not  only  in  rivers, 
lakes,  and  oceans,  but  in  the  earth  and  the  atmosphere 
(as  vapor).  Water  is  evaporated  from  the  ocean,  etc., 
and  taken  into  the  air  as  vapor;  here  it  is  condensed 
and  falls  to  earth  as  rain.  Some  of  this  water  sinks 
into  the  earth,  flows  along  on  hard  rock  as  underground 
streams,  and  later  comes  to  the  surface  again  through 
springs  or  wells.  Mineral  waters  are  found  in  streams 
that  dissolve  some  of  the  rock  through  which  they  flow. 
Rivers  carry  much  material  from  the  surface  of  the  land 
to  the  ocean ;  this  has  been  going  on  for  so  long  that 


COMPOUNDS  203 

the  ocean  water  contains  nearly  3^  of  mineral  salts  in 
solution.  A  large  part  of  this  dissolved  matter  is  com- 
mon salt  (NaCl) ;  the  limestone  (CaCO3)  in  » 
ocean  water  furnishes  material  for  the  shells 
of  many  small  sea  animals. 

In  chemistry  H2O   is   of  great  value.    It 
dissolves  more  different  things  than  any  other 
liquid,  and  as  it  is  a  neutral  compound  (not 
acting  chemically  upon   the    substance    dis- 
solved in  it),  water  is  a  very  useful  solvent.    To 
plant  life  H2O  is  of  first  importance.    Plants 
absorb  large  quantities  of  water  through  their 
roots ;  in  their  leaves  and  bark  some  of  it  is     FlG- 134 
combined  with  carbon,  making  the  great  bulk  of  the 
solid  matter.    Water  is  hardly  less  important  in  animal 
life.    It  is  present  in  nearly  all  foods,  and  all  parts  of 
the  animal  body  contain  a  great  deal  of  it. 

Experiment  128.  —  Arrange  apparatus  for  making  H,  as  in 
Experiment  121.    Heat  the  glass  tube  and  draw  it  out  to  a  small 
opening  (Fig.  134).    Into  the  test  tube  put  Zn  and  HC1 
to  make  H.    When  the  gas  has  flowed  a  few  seconds,  col- 
lect some  in  a  small 
test  tube  (by  hold- 
ing the  tube  mouth 
downward  over  the 
end  of  the  glass  de- 
livery tube);  touch  a 

lighted  match  to  the 

FIG.  135  ,    .     ... 

gas  collected  in  this 

small  test  tube.    If 

a  slight  explosion  occurs,  wait  a  moment  and  then  repeat ;  if  the 
gas  only  burns  quietly,  then  light  the  gas  escaping  from  the 
delivery  tube.  This  gas  is  H,  now  burning  in  air  (containing  O). 


204  COMMON  SUBSTANCES 

At  once  hold  a  cool  glass  beaker  or  tumbler  over  the  flame 
(Fig.  135)  and  note  the  condensing  of  water  vapor  upon  it. 
This  water  vapor  is  given  off  when  the  H  unites  with  the 
O,  —  H2  +  0  =  H20. 

236.  Sulphuric  Acid. — Sulphuric  acid,  H2SO4,  is  made 
in  great  quantities  by  the  union  of  the  elements  of  SO2, 
H2O,  and  O.    Being  a  very  strong  acid,  it  is  used  to 
break  up  the  salts  of  many  other  acids,  setting  those 
acids  free.     In  this  way  HC1  is  made  from  NaCl,  HNO3 
from  NaNO3,  etc.    H2SO4  forms  with  different  metals  an 
important  group  of  salts  called  sulphates. 

237.  Carbon  Dioxide.  —  Whenever  carbon  is  burned  in 
a  good  supply  of  air,  a  gas  called  carbon  dioxide  (CO2) 
is  formed.    CO2  is  a  colorless,  odorless  gas  ;  it  is  heavier 
than  air,   and  is  sometimes   called   carbonic   acid  gas. 
In  the  earth  CO2  occurs  widely  in  carbonates,  chiefly  as 
CaCOg  (limestone,  marble,  etc.).    It  is  given  off,  in  its 
free  gaseous  state,  from  burning  wood,  coal,  kerosene, 
illuminating  gas,  etc.  ;  also  from  the  lungs  of  animals, 
mixed  with  the  air  breathed  out.    Carbon  dioxide  occurs 
(in  a  very  small  quantity)  in  the  atmosphere,  where  it 
forms  an  important  part  of  the  food  of  plants.    The  gas 
which  causes  the  "  lightness  "  of  bread  and  cakes  is  gen- 
erally CO2,  and  the  same  gas  causes  the  effervescence 
of  soda  water  and  bottled  tonics. 

Experiment  129.  —  Into  a  large  test  tube  put  a  few  bits  of  mar- 
ble, and  add  HC1.  Stop  the  test  tube,  running  a  delivery  tube  to 
the  bottom  of  a  loosely  covered  jar,  as  in  Experiment  125.  When 
the  gas  has  flowed  freely  for  two  or  three  minutes  remove  the  tube 
from  the  jar,  carefully  covering  the  latter.  In  this  way  fill  two 
jars.  The  gas  is  CO2.  CaCO3  +  2  HC1  =  CaCl2  +  H2O  +  CO2. 


COMPOUNDS 


205 


Experiment  130.  —  Carefully  balance  a  thin  glass  beaker  on  a 
delicate  set  of  scales.  The  beaker  of  course  contains  air.  Now 
pour  the  CO2  from  one  jar  into  the  beaker,  as  in  Fig.  136.  If  the 
balance  is  not  changed,  repeat  the  experiment  carefully.  Compare 
the  weights  of  CO2  and  air. 

Experiment  131.  —  Into  the  other  jar  of  CO2  thrust  a  lighted 
stick  or  taper,  and  note  what  happens.  What  does  this  show  with 
regard  to  CO2?  Try  to  explain  why 
the  gas  should  behave  in  this  way. 

Carbon  dioxide  is  not  a 
direct  poison  to  animals,  but 
because  it  does  not  supply 
the  free  O  that  they  must 
breathe,  animals  cannot  live 
in  it.  For  the  same  reason  it 
is  injurious  to  man.  Oil  and 
gas  heaters  give  out  large 
quantities  of  CO2,  using  up 
the  O  from  the  air;  they 
should  not  be  used  in  rooms 
unless  a  constant  supply  of  fresh  air  is  possible.  CO2 
neither  burns  nor  supports  combustion. 

When  C  is  burned  with  a  poor  supply  of  air,  another 
gas  is  formed,  called  carbon  monoxide  (CO).  This  gas 
is  very  poisonous  to  man,  even  in  small  amounts.  It 
is  often  formed  in  coal  fires,  from  which  it  may  be 
given  off ;  hence  the  danger  of  sleeping  in  rooms  with 
a  coal  fire. 

238.  Ammonia.  —  Ammonia  is  a  compound  of  N  and 
H,  its  symbol  being  NH3.  It  is  formed  when  certain 
animal  matter  decomposes  in  air,  though  it  is  generally 


FIG.  136 


206  COMMON  SUBSTANCES 

formed  when  coal  is  distilled  in  making  illuminating 
gas.  NH3  is  a  gas,  but  it  dissolves  very  easily  in  water. 
A  solution  of  NH3  in  water  forms  NH4OH,  ammonium 
hydrate.  This  is  a  strong  alkali;  somewhat  weakened 
with  more  water,  it  is  used  as  "household  ammonia." 
Two  salts  of  this  base  are  common,  —  NH4C1,  famil- 
iarly called  sal  ammoniac,  and  NH4NO3,  ammonium 
nitrate.  A  solution  of  sal  ammoniac  in  water  is  used  in 
many  kinds  of  voltaic  cells. 

239.  Cellulose.  —  Cellulose  may  be  described  as  the 
chief  substance  which  makes  up  the  structure  of  plants. 
It  is  found  in  many  different  forms,  though  its  chemical 
composition  does  not  change.    All  wood  fiber,  trunks  of 
trees,  their  branches,  roots,  stems,  veins  of  leaves,  and 
parts  of  fruits  are  composed  largely  of  cellulose ;  also 
such  fibers  as  cotton,  flax,  and  hemp.    The  substance 
cellulose  is  a  carbohydrate  (§  219)  having  the  symbol 
C6H10O6.    It  is  formed  by  the  activity  of  the  plants, 
largely  from  the  H2O  and  CO2  that  the  soil  and  the 
atmosphere  furnish. 

240.  Starch.  —  Starch  is  a  carbohydrate  having  the 
symbol  C6H10O5.    It  is  made  by  the  action  of  plants, 
and  is  found  throughout  the  vegetable  world ;  seeds  of 
all  sorts  contain  starch,  and  some  plants  store  up  large 
masses  of  it,  as  sago  and  tapioca.    Starch  is  prepared  in 
large  quantities  from  corn  and  from  potatoes.    It  forms 
an  important  food  for  man,  both  in  its  prepared  state 
and  as  cereals,  —  barley,  oats,  wheat,  rye,  rice,  etc.    In 
cold  water,  starch  is  usually  not  soluble ;  but  in  hot 
water  it  partly  dissolves,  forming  a  paste. 


COMPOUNDS  207 

Notice  that  starch  has  the  same  chemical  composition 
as  cellulose  (C6H10O5).  The  chief  difference  between 
the  two  compounds  is  that  cellulose  is  the  actual  sub- 
stance of  which  the  plant  is  composed,  while  starch  is 
food  stored  by  the  plant  for  some  future  use.  Thus 
seeds  sprout,  and  the  young  plant  grows  for  a  short 
time  by  using  the  starch  stored  in  the  seed.  The  starch 
in  a  potato  serves  the  same  purpose. 

241.  Cane  Sugar.  —  Common  sugar  occurs  in  several 
vegetable   substances.    It   is    generally   obtained   from 
sugar  cane  or  beets.    The  cane  or  beet  is  usually  cut 
up  and  bruised  under  water,  the  sugar  being  dissolved 
out ;  the  solution  of  water  and  sugar  is  drawn  off  and 
boiled  to  a  syrup.    As  this  syrup  cools  some  of  the 
sugar  forms  in  crystals ;  these  are  dried  and  crushed  to 
make    granulated   sugar.    The  liquid   that  remains  is 
boiled  over,  and  again  cooled;  the  crystals  that  now 
form  are  called  brown  sugar.    After  boiling  the  liquid 
two  or  three  times  more,  no  crystals  will  form  and  the 
syrup  is  then  called  molasses. 

Cane  sugar  is  a  carbohydrate,  its  symbol  being 
C12H22On.  Its  uses  are  too  well  known. 

242.  Dextrose.  —  Many  fruits,  such  as  grapes,  plums, 
peaches,  etc.,  owe  their  sweetness  to  another  carbohy- 
drate, dextrose  (C6II12O6).    This  substance  is  sometimes 
called  glucose,  grape  sugar,  etc.    It  can  be  made  from 
cane  sugar  and  is  made  in  large  quantities  from  starch. 
Dextrose  is  about  three  fifths  as  sweet  as  cane  sugar. 
In  fruits  it  forms  an  easily  digested  food.    Confectioners 
use  a  great  deal  of  the  dextrose  that  is  made  from  starch. 


208  COMMON  SUBSTANCES 

243.  Alcohol. — Common  alcohol  (C2H5OH)  is  formed 
when  dextrose  or  grape  sugar  ferments  (§  267).    Hence 
it  often  appears  when  fruit  juices  are  allowed  to  stand 
for  some  time.    As  a  solvent,  alcohol  is  much  used  in 
making  varnishes,  'tinctures,  perfumes,  and  drugs.    It 
is  useful  in  medicine  because  it  stimulates  the  action 
of  certain  parts  of  the  body;  but  a  continual  use   of 
alcohol  in  any  quantity  is  injurious.    It   burns    with 
a   hot,  smokeless  flame,  being  thus  useful  in  several 
of  the  arts. 

244.  Fats  and  Oils.  —  Fats  and  oils  are  salts  formed 
by  the  union  of  glycerin  with  different  acids.    Fats  are 
solid  substances,  and  occur  usually  in  animal  matter. 
Oils  are  liquids ;  they  occur  in  both  plant  and  animal 
matter.    The  acids  that  may  unite  with  glycerin  to  form 
fats  or  oils  are  called  fatty  acids. 

245.  Soap.  —  A  soap  is  an  alkali  salt  of  a  fatty  acid. 
Soap  may  be  made  by  boiling  fats  together  with  an 
alkali  (NaOH  or  KOH).    The  fats  break  up  into  their 
acids  and  glyceryl,  the  metal  of  the  alkali  uniting  with 
the  acid  to  form  soap,  and  glycerin  being  given   off. 
Thus  soap  is  a  salt  of  the  metal  Na  or  K  with  an  acid 
obtained  from  a  fat. 

Soap  acts  upon  the  oily  matter  that  is  mixed  with 
dirt  on  the  skin  or  on  fabrics ;  the  oil  is  broken  up  into 
tiny  particles  that  may  be  washed  away  by  water,  carry- 
ing the  dirt  with  them.  Water  alone  could  not  do  this, 
because  fats  and  oils  do  not  dissolve  in  it.  Many  kinds 
of  soap  contain  much  free  alkali ;  this  renders  them  effect- 
ive for  cleansing,  but  often  injurious  to  certain  fabrics. 


COMPOUNDS  209 


QUESTIONS 

1.  Of  what  is  water  composed?    What  is  its  symbol? 

2.  What  are  mineral  waters  ?    Why  is  ocean  water  salt  ? 

3.  Of  what  use  is  water  in  chemistry?    Why  is  water  a 
valuable  solvent?    State  what   you  can   of   the   importance   of 
water  in  living  bodies. 

4.  What  is  the  symbol  for  sulphuric  acid?    How  and  why 
is  sulphuric  acid   used  in  obtaining   other  acids?    What  is  a 
sulphate  ? 

5.  How  is  carbon  dioxide  formed?    Describe  the  compound. 
Name  any  common  occurrence  of  carbon  dioxide.    Is  CO2  useful 
to  plants  ?    Is  it  useful  to  animals  ? 

6.  What  is  the  objection  to  oil  stoves  and  gas  heaters  in  a 
room?    Does  CO2  support  combustion? 

7.  Under  what  conditions  is  carbon  monoxide  formed  ?    Why 
is  it  more  dangerous  than   CO2?    State  the  danger  from  coal 
stoves. 

8.  Of  what  is  ammonia  composed?    What  is  the  substance 
that  is  commonly  called  "  household  ammonia  "  ?    Name  two  salts 
of  NH4OH. 

9.  What   is   cellulose?    State   its   symbol.    Name   parts   of 
plants  that  are  composed  largely  of  cellulose. 

10.  Of  what  is  starch  composed ?    How  is  it  made?    Where  is 
it  found  ?    Of  what  use  is  starch  in  seeds  ?    Of  what  use  in  other 
parts  of  plants  ?    What  is  the  use  of  starch  to  man  ? 

11.  From  what  is  cane  sugar  obtained  and  by  what  means? 
What  is  its  symbol?    Wrhat  is  molasses? 

12.  Where  does  dextrose  occur?    What  other  names  are  given 
to  it  ?    From  what  other  compounds  can  it  be  made  ? 

13.  State  uses  of  common  alcohol.    How  is  it  formed? 

14.  What  are  fats  and  oils  ?    Where  is  each  found  ?    What  is 
a  fatty  acid  ? 

15.  What  is  a  soap?    Explain  how  soap  is  formed.    Show  the 
action  of  soap  in  cleansing  things.    Why  does  not  water  alone 
serve  as  well  ? 


210 


COMMON  SUBSTANCES 


SECTION    III 
MIXTURES 

246.  Air.  —  The  air  is  a  mixture  of  gases,  the  quan- 
tities of  which  may  vary  somewhat.  Pure  air  usually 
contains  about  four  fifths  nitrogen  and  one  fifth  oxygen, 
besides  a  very  small  amount  of  carbon  dioxide.  In  the 
atmosphere  there  is  also  more  or  less  water  vapor  all 

the  time,  the  quan- 
tity varying  greatly 
at  different  times  and 
places.  These  four 
substances,  N,  O,  CO2, 
and  H2O,  are  each  of 
some  important  use 
in  the  air;  but  there 
are  also  other  gases, 
such  as  H,  Cl,  H2S, 
NH3,  etc.,  which  mix 
with  the  atmosphere 
in  very  small  amounts 
now  and  then.  The 
quantity  of  such  im- 
purities in  the  air  is  generally  greater  in  cities,  near  fac- 
tories or  chemical  works,  near  marshes  and  swamps,  in 
mines,  etc.  Air  in  the  country  or  over  the  sea  is  usually 
more  nearly  pure,  though  no  strict  rule  can  be  stated. 

Experiment  132 Float  a  cork  on  water  in  a  large  vessel. 

Place  a  bit  of  P  on  the  cork  and  light  it ;  at  once  cover  the  cork 
with  a  large  jar,  as  in  Fig.  137,  holding  the  mouth  of  the  jar 
under  water  all  the  time.  Allow  the  P  to  burn  as  long  as  it  will, 


FIG.  137 


MIXTURES  211 

carefully  holding  the  jar.  As  the  P  burns  (combining  with  O), 
the  oxygen  that  was  in  the  jar  is  used  up,  leaving  the  nitrogen 
nearly  pure.  The  O  combines  with  the  P,  the  compound  then 
being  dissolved  in  the  water.  Thus  the  air  in  the  jar  loses  from 
its  total  volume  the  volume  of  the  O  that  was  in  it  at  first,  and 
the  water  rises  into  the  jar  to  take  its  place.  Compare  the  volume 
of  water  that  so  rises  with  that  of  the  gas  still  left  in  the  jar. 
Of  what  is  this  gas  largely  composed  now  ? 

Experiment  133.  —  Carefully  cover  the  jar  under  water  ;  then 
lift  it  out  and  set  it  right  side  up  on  the  table.  Carefully  uncover, 
at  once  thrusting  a  lighted  splinter  into  the  jar  which  is  now 
filled  with  N.  Does  N  support  combustion  ? 

The  use  of  O  in  the  atmosphere  is  to  help  support 
animal  life,  and  to  support  combustion.  Nitrogen,  being 
very  inactive,  serves  to  check  too  strong  an  action  of  O ; 
in  an  atmosphere  of  pure  O  fires  would  burn  beyond 
control  and  animals  could  not  live.  The  use  of  CO2  in 
the  air  is  to  supply  to  plants  the  C  that  they  need  in 
making  starch  and  cellulose.  Water  vapor  in  the  air 
serves  to  temper  the  climate  of  some  places,  and  is  very 
important  in  furnishing  rain.  Without  evaporation  and 
rainfall  the  soil  would  everywhere  become  dry  and  the 
water  would  slowly  drain  from  the  land  into  the  ocean. 

247.  Soil.  —  The  earth  is  thought  to  be  composed  of 
solid  rock.  A  great  deal  of  its  surface  is  covered  with 
a  layer  of  loose  earthy  matter  called  soil,  which  varies 
in  thickness  from  an  inch  or  two  to  several  hundred 
feet  in  some  places.  On  the  average  the  soil  is  but  a 
few  feet  deep.  Soil  is  made  of  tiny  particles  of  rock 
that  have  been  worn  off  from  the  solid  rock  mass  in 
different  ways.  As  the  kinds  of  rock  that  have  been 
thus  worn  are  many,  so  we  find  many  different  kinds  of 


212  COMMON  SUBSTANCES 

soils.  In  many  places  decayed  plant  and  animal  sub- 
stances have  mixed  with  the  soil,  making  more  or  less 
change  in  its  composition. 

The  soil  is  very  necessary  to  plant  life.  It  allows  the 
roots  a  good  support ;  it  holds  moisture  which  the 
plant  may  slowly  absorb  ;  and  it  supplies  small  amounts 
of  mineral  matter,  which  is  dissolved  by  the  water  and 
so  taken  into  the  plant  structure. 

248.  Earthenware  and  Porcelain.  —  Clay  is  a  kind  of 
soil  that  contains  a  large  quantity  of  aluminium  silicate 


(§  234).  Clay  may  be  moistened  slightly  and  then 
molded  into  different  shapes.  If  it  is  then  baked  in  a 
furnace  for  some  time  the  silicate  becomes  hard  so 
that  the  vessel  will  keep  its  shape.  In  this  way  brick 
and  vessels  of  earthenware  (Fig.  138)  are  made. 

Sometimes  the  aluminium  silicate  is  found  pure.  If 
this  be  treated  similarly  to  the  clay,  a  finer  grade  of 
ware  will  be  made  ;  this  is  called  porcelain. 

249.  Glass.  —  Grlass  is  a  mixture  of  silicates  of  two  or 
more  metals,  —  usually  Ca,  Na,  K,  Pb,  Al,  or  Fe.  It  is 


MIXTURES  213 

formed  by  heating  white  sand  (silica,  SiO2)  together 
with  a  compound  of  each  of  the  metals  to  be  used. 
Sand  alone  will  not  melt,  but  when  heated  with  these 
other  compounds  they  all  melt  and  unite  together, 
forming  the  silicates.  This  heated  mass  must  then  be 
cooled  slowly,  to  make  the  glass  as  tough  as  possible. 

The  kind  of  glass  formed,  depends  upon  what  metals 
are  used.  Window  glass  is  a  silicate  of  Ca  and  Na; 
bottle  glass  is  a  silicate  of  Ca,  Na,  Al,  and  Fe  ;  glass 
used  for  lenses  is  a  silicate  of  K  and  Pb.  Very  few  sub- 
stances act  upon  glass ;  air  does  not  affect  it,  and  liquids 
do  not  generally  pass  through  it.  For  these  reasons 
it  is  very  useful  as  material  from  which  to 
make  bottles,  jars,  and  other  vessels.  It  is 
also  one  of  the  very  few  transparent  solids 
that  are  common.  Its  importance  is  great. 


250.  Wood. — In  general,  wood  is  made  up 
mostly  of  cellulose  ;  the  chief  elements  that 
it  contains  are  therefore  C,  H,  and  O.    Mixed 
with  the  cellulose  are  usually  small  quanti- 
ties of  mineral  salts ;  these  are  left  as  ashes 
after  wood  is  burned.    Living  wood  always 
contains   some   water  ;   and  many  kinds  of      FlG- 139 
wood  contain  other  substances,  such  as  oils,  acids,  pitch, 
resin,  balsams,  etc. 

The  cellulose  in  wood  usually  occurs  in  the  form  of 
fibers.  In  a  cornstalk  the  fibers  may  be  seen  singly 
(Fig.  139),  but  they  are  more  commonly  grouped 
together  in  masses.  The  sap  flows  in  spaces  between 
the  masses  of  fibers. 


214 


COMMON  SUBSTANCES 


Experiment  134.  —  Secure  a  stick  of  oak  wood,  cut  for  a  stove. 
Split  it  lengthwise  ;  examine  the  freshly  opened  surface,  using  a 
microscope  if  possible.  Can  you  see  fibers?  Do  you  see  groups 
of  fibers  ?  Examine  the  end  of  the  stick,  looking  for  masses  of 

fibers.    Do  you  see  the  openings 
'•'  ~^;>  for  the  sap  ? 

Experiment  135 Put  some 

bits  of  dry  wood  into  a  test 
tube  and  apply  heat  (Fig.  140). 
When  the  wood  is  thoroughly 
black,  cease  to  heat  it.  Examine 
the  remains.  What  is  left  in  the 
tube  and  what  has  gone  from 
the  wood  ? 


FIG.  140 


251.  Cotton  Cloth.  —  Sev- 
eral plants  — cotton,  flax,  and 
hemp  —  produce  growths  of 
fine  fibers  that  may  be  easily  separated.  These  fibers 
are  twisted  together  to  form  rope,  twine,  and  small 
threads.  Threads  of  cotton  or  linen  are  woven  together 
to  form  different  sorts  of  cotton  and  linen  fabrics.  Plant 
fibers  of  this  sort  are  the  same  in  chemical  structure  as 
the  fibers  of  wood  ;  they  are  therefore  cellulose. 

252.  Paper.  —  Paper  is  a  mass  of  fibers  very  firmly 
pressed  together.  The  fibers  are  obtained  from  cotton 
or  linen  rags  and  from  wood  pulp  (a  mass  of  short 
separated  wood  fibers). 

To  make  paper,  the  rags  are  cleansed  and  then  torn  and 
shredded  into  tiny  fibers  by  machines.  Water  is  then 
added  to  the  mass  of  fibers,  so  that  the  mixture  easily 
flows.  This  mixture  is  poured  out  upon  a  flat  surface, 
made  so  that  the  water  may  partly  drain  out,  leaving 


MIXTURES  215 

the  fibrous  mass  spread  evenly.  An  endless  strip  of 
cloth  now  picks  up  this  fibrous  mass,  carrying  it  through 
several  rollers.  After  passing  through  these,  the  paper 
is  strong  enough  to  go  on  through  several  more  rollers 
without  the  cloth.  The  rollers  are  heated,  and  they 
press  the  fiber  so  firmly  together  that  the  mass  becomes 
paper.  Wood  pulp  is  often  mixed  with  the  fibers  from 
rags  ;  some  cheaper  papers  are  made  largely  of  wood 
pulp.  Since  it  is  made  from  wood  and  cloth  fibers, 
paper  also  must  be  mostly  cellulose. 

Experiment  136 . — Into  one  crucible  put  a  mass  of  cotton 
threads,  and  into  another  some  bits  of  paper.  Cover  each  sub- 
stance with  a  little  dry  sand,  heat  for  a  few  minutes,  and  examine. 

253.  Coal.  —  Millions  of  years  before  man  appeared 
on  earth  plants  grew  upon  its  surface.  In  some  places 
large  masses  of  trees,  leaves,  ferns,  and  other  plant 
forms  were  piled  up  as  they  died,  and  were  later  cov- 
ered by  layers  of  soil  and  rock. 
These  slowly  decomposed,  much 
of  the  gaseous  part  of  the  wood 
passing  off,  but  the  carbon  remain- 
ing. In  time  these  masses  became 
hard,  and  to-day  we  find  them  in 
the  earth  as  coal.  Thus  we  see  FIG.  141 

that  coal  contains  the  same  elements  that  are  found  in 
wood,  but  the  gaseous  elements  are  much  less  in  quantity 
and  the  C  largely  remains. 

Anthracite  contains  more  carbon  and  less  of  other 
elements  than  soft  coal.  Some  anthracite  is  over  nine 
tenths  carbon ;  it  is  the  hard  kind,  such  as  is  burned  in 


216 


COMMON  SUBSTANCES 


stoves  (see  Fig.  141).  Bituminous  coal  is  a  softer  vari- 
ety. It  contains  more  gases,  burns  at  a  lower  tempera- 
ture, and  shows  much  more  flame  while  burning  than 
does  hard  coal. 


254.  Illuminating  Gas.  —  If  soft  coal  (bituminous)  be 
heated    to  a   high  degree  without   any  supply  of  air, 
the  coal  will  be   decomposed,  its   elements 
combining  and  mixing  with  each  other  to 
form  new  substances.    The  solid  substance 

that   remains 

IB  is  nearly  pure 

C  ;  it  is  called 
coke.  The  liq- 
uids unite  in 
a  mixed  mass 
called  coal 
tar.  The  gases 
that  are  given 
off  are  first 
passed  through 
water,  which 
dissolves  the 

ammonia  (NH3)  and  thus  removes  it.  The  gases  which 
remain  in  the  mixture  form  illuminating  gas.  This 
contains  some  free  hydrogen  and  some  compounds  of 
hydrogen  and  carbon.  So  we  see  that  illuminating  gas 
contains  largely  the  elements  C  and  H,  both  of  which 
burn  in  air. 

The  process  is  carried  on  in  gas  works.    Coal  is  put 
into  large  iron  retorts  (a,  Fig.  142)  and  heated  by  a 


FIG'142 


MIXTURES  217 

fire,  b,  placed  underneath.  Note  that  the  coal  itself  is 
not  burned  but  only  heated  without  air  until  it  is 
decomposed. 

255.  Petroleum.  —  Petroleum  is  an  oily  liquid  found 
in  the  earth  in  some  places.    Pennsylvania  and  Texas 
have  large  oil  fields.    Petroleum  contains  many  hydro- 
carbons (§  218).    Among  the  useful  mixtures  obtained 
from   it   are    kerosene,    benzine,  gasoline,  naphtha,   and 
paraffin.     Candles  are   commonly  made  from  paraffin. 
Note  that  each  of  these  substances  contains  largely  the 
elements  C  and  H. 

256.  Coal  Tar.  —  In  the  process  of  making  illuminat- 
ing gas  from  coal,  a  thick  black  liquid  called  coal  tar  is 
formed  (§  254).    This  liquid  has  been  found  to  contain 
a  great  number  of  compounds,  so  that  coal  tar  is  now 
the  source  of  many  common  and  important  substances. 
Among  these  we  may  mention  phenol,  or  carbolic  acid  ; 
saccharine,  a  substance  that  is  far  sweeter  than  sugar ; 
aniline  dyes  of  many  shades ;  and  various  essences  and 
perfumes.    The  compounds  found  in  coal  tar  contain 
chiefly  the  elements  C,  H,  0,  and  N. 

257.  Explosives.  —  Gunpowder  and  other   explosives 
are   mixtures    of   such  substances    as   may  easily  and 
quickly  act  upon  each  other  so  as  to  produce  a  large 
volume  of  gas.    Explosives  are  generally  either  solids 
or  liquids.    Under  a  slight  impulse  (a  spark  or  a  sudden 
jarring)  they  quickly  form  into  gases.    These  gases  nat- 
urally take  up  far  more  room  than  the  solid  or  liquid 
mass,  and  in  expanding  to  their  natural  volume   they 
exert  great  force. 


218 


COMMON  SUBSTANCES 


G-unpowder  is  a  mixture  of  saltpeter  (KNO3),  carbon, 
and  sulphur.  Upon  exploding,  the  gases  N  and  CO2 
are  set  free.  Gf-un  cotton  is  a  nitrate  of  cellulose  chiefly. 
Glycerin  also  unites  with  HNO3,  forming  a  nitrate 
known  as  nitroglycerin.  Nitroglycerin  is  a  pale  yellow 
liquid,  highly  explosive.  It  is  used  in  making  dynamite 

and  some  other  explo- 
sives. Note  that  these 
substances  all  contain 
some  nitrogen;  because 
N  is  so  inactive  it  forms 
compounds  that  easily 
break  up  and  set  the 
N  (gas)  free. 


FIG.  143 


Experiment    137.  —  In 

a  mortar  mix  12  grams 
KNO3,  2  g.  of  C  (charcoal), 
and  \%  g.  of  S.  When  thoroughly  mixed,  put  a  small  amount 
on  a  piece  of  metal  and  touch  it  with  a  lighted  match.  Notice 
how  it  burns.  The  mixture  is  gunpowder.  Why  does  it  not 
explode  with  a  loud  report  ?  Try  to  burn  some  gunpowder  by  con- 
verging the  sun's  rays  upon  some  one  spot,  as  in  Experiment  94 
(see  Fig.  143). 

258.  Foods.  —  Animal  bodies  are  made  up  of  the  same 
elements  that  compose  matter  in  general,  and  only  a 
few  of  these  elements  are  present  in  any  quantity. 
Since  animals  grow  by  taking  in  food,  we  can  get  a 
good  idea  of  what  elements  are  most  needed  by  study- 
ing the  foods  used.  Man  is  supplied  with  food  that  is 
largely  of  either  animal  or  plant  growth ;  but  since  the 
animals  eat  either  plants  or  other  animals  that  may  live 


MIXTURES  219 

upon  plant  growths,  we  see  that  nearly  all  of  our  food 
comes  from  the  soil  in  the  first  place. 

Man's  foods  may  be  divided  into  five  general  classes. 
First  of  all  is  water,  which  is  needed  in  all  parts  of  the 
body,  and  of  which  man  uses  a  large  amount.  Next  in 
quantity  are  the  carbohydrates  (§  219)  composed  of  C, 
H,  and  O ;  these  supply  energy  and  heat  to  the  body. 
Proteids  (§  220)  contain  C,  H,  O,  and  N ;  they  serve 
to  build  up  muscle  and  other  parts.  Small  quantities 
of  fats  serve  to  give  energy  to  the  system.  Last  of 
all  are  the  salts,  of  which  many  occur  in  small  quan- 
tities in  other  foods.  The  elements  P,  Cl,  S,  Ca,  Na, 
K,  Fe,  etc.,  are  taken  on  in  slight  amounts  as  salts. 

259.  Fuels.  —  The  substances  commonly  used  as 
fuels  have  already  come  to  our  attention ;  among  them 
we  recall  wood,  coal,  illuminating  gas,  kerosene,  gaso- 
line, naphtha,  benzine,  and  alcohol.  Other  things  less 
commonly  used  as  fuels  are  paper,  rags,  straw,  and  peat 
(partly  decomposed  vegetable  matter).  In  all  these  sub- 
stances note  that  the  elements  hydrogen  and  carbon  are 
present ;  both  of  these  burn  in  air  (i.e.  combine  with  O). 

QUESTIONS 

1.  What  four  substances  does  air  usually  contain  ?    State  the 
uses  of  each  of  these.    In  what  proportion  does  the  N  and  the  O 
occur  ? 

2.  Of  what  is  the  soil  generally  composed  ?    By  what  different 
means  may  it  have  been  formed?    Name  any  common  uses  of 
the  soil. 

3.  How  are  earthenware  vessels  made  ?    Of  what  is  porcelain 
made? 


220  COMMON  SUBSTANCES 

4.  What  is  glass?    Of  what  substances  is  glass  made?    How 
are  these  compounds  treated  to  form  glass?    Name  as  many  uses 
of  glass  as  you  can.    What  two  properties  make  it  valuable? 

5.  Of  what  substance  is  wood  largely  composed  ?    What  ele- 
ments are  present  in  this  substance?    Name  other  things  that 
occur  in  some  woods.    Of  what  are  ashes  formed  ? 

6.  From  what  is  cotton  cloth  made  ?    What  then  is  the  chemi- 
cal composition  of  cotton  cloth,  i.e.  what  elements  are  present? 

7.  Of  what  is  paper  made  ?    Describe  the  process  of  making 
paper.    What  elements  does  it  contain  ? 

8.  Of  what  is  coal  formed?    How  was  it  formed?    What 
element  constitutes  the  larger  part  of  hard  coal  ?  What  other  ele- 
ments are  present?    Distinguish  anthracite  and  bituminous  coal. 

9.  Describe  the  making  of    illuminating  gas.    What  other 
substances  are  formed  at  the  same  time  ?    Of  what  elements  is 
illuminating  gas  largely  composed? 

10.  What    is    petroleum?    What    substances    are    obtained 
from  it? 

11.  Name  some  important  substances  that  are  formed  from 
coal  tar. 

12.  What   sort   of   a   mixture   is    an   explosive?    Show   how 
explosive  mixtures  may  be  used  to  exert  great  force.    State  the 
composition  of  gunpowder  ;  of  gun  cotton ;  of  nitroglycerin. 

13.  What  are  the  five  classes  of  foods  used  by  man?    Name 
some  elements  that  are  common  in  the  body.    Of  what  use  are 
carbohydrates  in  the  system  ? 

14.  Name  some  common  fuels.    What  two  elements  do  they 
all  contain  ? 


CHAPTER   X 
COMMON  CHEMICAL  PROCESSES 

260.  Combustion.  —  Combustion  is  a  chemical  union 
which  takes  place  rapidly,  giving  off  light  and  heat.  The 
word  fire  is  commonly  used  instead  of  combustion.  Two 
things  are  necessary  in  order  that  combustion  may  take 
place  —  a  substance  to  burn  (called  a  combustible)  and 
a  substance  with  which  it  may  unite.  The  latter  sub- 
stance is  said  to  support  the  combustion.  We  have 
learned  that  the  things  commonly  burned  as  fuels  con- 
tain the  elements  C  and  H  (§  259)  ;  also  that  the  great 
supporter  of  combustion  in  the  air  is  O  (§222).  With 
these  facts  in  mind,  it  will  be  seen  that  the  most  com- 
mon fires  are  simply  the  rapid  union  of  carbon  and 
hydrogen  with  oxygen.  The  compounds  formed  by  this 
union  will  be  carbon  dioxide  (CO2)  and  water  vapor 
(H,0). 

Now  it  is  well  known  that  in  order  to  make  any 
substance  burn,  heat  must  be  applied.  In  other  words, 
oxygen  will  not  easily  unite  with  other  substances 
unless  their  temperature  be  raised.  The  temperature 
at  which  different  substances  will  burn  in  air  varies 
greatly ;  carbon,  for  example,  needs  a  greater  degree  of 
heat  than  hydrogen,  while  matches  containing  phos- 
phorus may  be  sufficiently  heated  by  simply  rubbing 
them.  When  we  wish  to  start  a  fire,  however,  we  do 
not  heat  the  whole  mass  that  is  to  be  burned,  but  only 

221 


222  COMMON  CHEMICAL  PROCESSES 

a  small  portion  of  it.  This  part  burns,  giving  off  heat; 
thus  the  parts  right  around  it  become  heated  until  they 
also  burn;  and  in  this  way  the  whole  mass  is  finally 
heated  and  burned. 

We  have  seen  that  in  order  for  any  substance  to  burn 
in  air,  it  must  be  heated  and  constantly  supplied  with 
oxygen.  Clearly,  then,  a  fire  may  be  stopped  by  cooling 
the  burning  mass  or  by  cutting  off  the  supply  of  oxygen 
(or  air).  Water  is  commonly  applied,  and  it  serves  both 
purposes  ;  but  water  is  not  always  the  best  thing  to  use. 
Chemical  fire  extinguishers  are  of  value  when  the  fire  is 

small  ;  they  are  usu- 
ally devices  for  mak- 
ing a  large  amount 
of  CO2  on  the  spot — 
and  CO2  does  not  sup- 
port combustion.  The 
FIG.  144  ,J[1L^  most  effective  way  to 

stop  a  fire  when  first 
started,  is  to  cover  it  closely  with  rugs,  clothing,  earth, 
flour,  or  any  solid  which  does  not  easily  burn;  in  this 
way  the  air  is  kept  away  and  the  fire  is  "  smothered." 

Experiment  138.  —  Using  a  long  or  circular  oil  burner,  turn  the 
wick  up  just  above  the  metal  and  light  it  at  one  point.  Note 
the  creeping  of  the  flame  along  the  wick  as  each  part  is  heated 
from  the  burning  portions. 

Experiment  139.  —  Try  to  set  fire  to  small  quantities  each  of 
wood,  alcohol,  charcoal,  sulphur,  kerosene,  phosphorus,  hydrogen, 
soft  coal,  etc.  (To  use  alcohol  and  kerosene,  pour  a  few  drops  on 
a  flat  piece  of  wick.)  Roughly  compare  the  temperatures  at 
which  these  substances  burn.  Other  things  could  be  used.  Be 
careful  with  phosphorus,  alcohol,  kerosene,  etc. 


EXPLOSION 


223 


FIG.  145 


Experiment  140.  —  Into  a  large  test  tube  fit  a  bent  delivery 
tube  (of  small  size),  as  in  Fig.  144.  Put  bits  of  marble  into  the 
test  tube  and  pour  upon  them  strong  HC1,  so  as  to  make  a  good 
flow  of  CO2  (Experiment  129).  Direct  this  stream  of  CO2  gas 
upon  a  candle  flame  or  a  small  fire  made  of  chips. 
Note  the  effect,  and  explain. 

Experiment  141.  —  Cut  holes  in  a  piece  of 
cardboard  and  fit  it  into  a  small  glass  chimney. 
Stick  a  lighted  candle  on  the  card  (Fig.  145). 
Now  cover  the  chimney  tightly  at  the  top  for  a 
moment.  Light  the  candle  and  set  the  chimney 
upon  some  flat  surface  that  will  close  it  at  the 
bottom.  Explain. 

261.  Explosion.  — An  explosion  is  a  sort 
of  combustion  that  takes  place  very  rap- 
idly in  a  confined  space.    Two  or  more 
substances  that  may  easily  unite  are  mixed  together ; 
a  mere  spark  at  some  point  in  the  mixture  will  often 
cause  action  throughout  the  whole  mass  in  a  moment. 
If  the  mixture  is  confined  in  a  small  space,  the  gases 
that  are  formed  by  the  chemical  action  will  have  so 
much  larger  natural  volume  that  they  will  expand  and 
burst  the  walls  that  confined  them. 

262.  Flames.  —  A  burning  gas  gives  rise  to  a  flame; 
burning  solids  usually  glow  and  are  luminous  (§  126), 
but  without  flame.    When  a  solid  substance,  such  as 
wood,  burns  with  a  flame,  it  is  because  the  substance  is 
being  decomposed  by  the  heat,  and  the  gases  that  are 
given  off  cause  the  flames. 

Experiment  142.  —  Make  some  H  as  in  Experiment  121,  using 
care  in  lighting  the  gas.  The  flame  is  usually  somewhat  colored 
by  solid  particles  from  the  heated  glass  tube  ;  but  if  the  end  be 


224  COMMON  CHEMICAL  PROCESSES 

fairly  large  or  protected  by  a  piece  of  platinum  (Fig.  146),  it 
may  be  possible  to  show  that  H  burns  with  a  colorless  flame. 

Experiment  143.  —  Pour  a  little  alcohol  over  a  few  bits 
of  charcoal  (carbon)  about  the  size  of  marbles.    Pile  these         A 
up  on  a  glass  or  metal  plate  and  light  the  mass.    When         I  I 
the  charcoal  is  well  kindled,  JJJ 

note  that  it  glows  and  gives    <.!•••  —         _.N.ig-.— —          f:^^^ 
off  light  but  no  flame.  What 
is   being   formed?    Why   is 
there  no  flame?    Do  not   try  this  without  the  teacher's  help. 

The  substances  commonly  burned  to  furnish  light  — 
illuminating  gas,  kerosene,  gasoline,  and  paraffin  can- 
dles—  are  made  up  mostly  of  the  gas  hydrogen  and 
the  solid  carbon.  Upon  being  lighted,  the  H  burns  and 
furnishes  the  flame,  while  the  C  in  tiny  particles  becomes 
heated  in  this  flame  and  glows,  so  that  the  whole  gives 
off  light.  When  a  lamp  "smokes  "  it  is  because  the  oil 
is  being  decomposed  and  the  carbon  particles  given  off 
faster  than  they  can  be  heated  and  burned  in  the  flame. 
In  any  case,  smoke  is  made  up  of  particles  of  matter  that 

were   not   consumed 
in  the  flame. 

Experiment  144. — 

Light  a  candle  and  trim 

it  to  give  a  good  flame. 

Hold  a  piece  of  earth- 
FlG- 147  enware  in  this  flame  as 

in  Fig.  147  ;  note  the  de- 
posit of  soot.  Of  what  is  it  composed?  The  solid  object  cools 
the  flame  so  that  it  does  not  consume  all  of  the  C  that  is  given 
off  from  the  wick. 

263.  Fire.  —  As  commonly  used,  the  word  fire  may 
easily  give  us  a  wrong  idea  of  its  meaning.    A  fire  is 


OXIDATION  225 

only  a  process  of  combustion,  and  the  word  fire  means 
all  that  combustion  means,  —  the  chemical  union  of  dif- 
ferent elements,  together  with  the  heat,  flame,  light, 
etc.,  that  may  occur  in  the  process.  A  little  thought 
will  make  the  matter  clear.  The  chemical  action  in  com- 
mon fires  is  between  the  elements  H  or  C  and  O  ;  the 
heat  is  given  off  as  from  any  chemical  union  (§  206) ; 
flames  show  that  a  gas  (usually  H)  is  burning ;  light  is 
given  off  from  glowing  solid  particles  (commonly  of 
C) ;  smoke  is  a  mass  of  solid  particles  that  were  not 
entirely  burned;  and  ashes  are  made  up  of  mineral 
matter  that  could  not  burn. 

264.  Oxidation.  —  We  have  learned  that  oxygen  com- 
bines directly  with  many  elements  (§  222),  and  that  it 
does  this  rapidly  if  they  be  heated  to  a  high  enough 
degree  (§  260).  Now  it  also  happens  that  several  ele- 
ments will  combine  with  oxygen  even  at  the  ordinary 
temperature  of  the  air,  but  they  do  this  very  slowly. 
The  process  is  called  oxidation;  the  compound  formed 
is  called  an  oxide. 

Experiment  145.  —  File  a  piece  of  iron  till  bright  ;  dip  it  in 
water,  remove  it,  and  without  even  shaking  off  the  drops  of  water, 
set  it  aside.  In  two  or  three  days  examine  it,  and  tell  what  has 
happened. 

Experiment  146.  —  Scrape  a  piece  of  lead  till  its  surface  is 
bright  and  clean,  then  set  it  aside.  In  a  few  days  examine  the 
lead,  note  its  surface,  and  explain  the  change. 

Most  of  the  metals  will  combine  directly  with  O ; 
gold  does  not,  and  silver  forms  a  sulphide  rather  than 
an  oxide  in  air.  The  presence  of  water  usually  assists 


226  COMMON  CHEMICAL  PROCESSES 

oxidation.  Iron  rust,  the  most  common  of  metallic 
oxides,  is  formed  by  the  union  of  iron  with  oxygen,  but 
this  is  always  greatly  helped  by  water,  even  if  only  the 
moisture  in  the  air. 

265.  Oxidation  in  Animal  Bodies We  breathe  air 

into  the  lungs  for  the  oxygen  that  it  contains.     Carbon  is 
taken  into  the  body  in  the  food  that  is  eaten  (§258), 
and  is  found  all  over  the  system.    In  the  lungs  O  is 
separated  from  the  air  and  is  carried  by  the  blood  to  all 
parts  of  the  body.    There  it  unites  with  the  C  which  is 
already  in  those  parts,  and  forms  CO2.    This  chemical 
union  of  the  C  with  O  gives  off  energy,  as  does  any 
chemical  union ;  the  energy  is  used  by  the  body,  partly 
as  heat  to  keep  us  warm,  and  partly  as  muscular  energy 
so  that  all  parts  may  move  and  do  their  work. 

The  CO2  that  is  formed  is  carried  to  the  lungs,  where 
it  leaves  the  body  in  the  air  that  is  breathed  out.  Plants 
take  air  into  their  leaves,  separate  the  C  from  the  O  of 
the  CO2,  use  the  carbon  in  making  starch,  and  give  out 
pure  oxygen  to  the  air  again. 

266.  Decay.  —  Many  substances,  particularly  of  plant 
and  animal  matter,  will  decay  after  a  time  unless  cared 
for  in  some  special  way.    The  signs  of  decay  are  many  : 
the  body  is  usually  soft  and  easily  crumbles ;,  it  is  gen- 
erally much  smaller  in  size  than  before ;  and  often  an 
odor  is  given  off.    The  smaller  size  is  due  to  the  fact 
that  a  large  proportion  of  any  animal  or  plant  matter  is 
of  gaseous  elements  ;  these  of  course  pass  off  when  they 
are  set  free  by  decay.    The  odor  is  caused  by  gases  that 
are  formed ;  one  of  the  most  common  of  these  gases  is 


FERMENTATION  227 

hydrogen  sulphide  (H2S),  —  a  compound  that  is  found  in 
large  amounts  in  eggs  that  have  lost  their  usefulness. 

Decay  is  a  process  of  decomposition  which  goes  on 
slowly  and  quietly.  Its  causes  are  not  well  understood 
in  all  cases,  but  it  is  thought  to  be  sometimes  due  to 
very  tiny  vegetable  forms  called  bac- 
teria. These  tiny  bodies  are  too  small 
to  be  seen  without  a  powerful  micro- 
scope ;  they  are  common  in  the  air, 
the  soil,  and  in  water,  as  well  as  in 
various  other  substances.  Heat  gen- 
erally kills  them,  and  most  kinds 
seem  to  work  best  in  a  good  supply 
of  oxygen.  Fruits  are  often  put  up  in  jars  while  hot 
and  at  once  covered  tightly;  in  this  way  they  may  be 
kept  for  a  long  time  without  decaying.  Fig.  148  shows 
several  bacteria,  greatly  magnified.  There  are  of  course 
other  causes  of  decay. 

267.  Fermentation.  —  If  apple  juice  is  allowed  to 
stand  for  a  time,  we  know  that  alcohol  may  form  in 
it  and  the  juice  becomes  cider;  similarly,  grape  juice 
may  become  wine,  containing  alcohol.  Clearly  a  chem- 
ical change  goes  on  in  the  liquid,  and  this  change  is 
called  fermentation.  It  is  caused  by  something  that  is 
present  in  the  fruit  juice,  or  that  gets  into  it  from  the 
air.  These  things  that  may  cause  fermentation  are 
called  ferments. 

Many  different  ferments  are  known,  and  they  act 
upon  many  substances.  One  of  the  most  common  of 
ferments  is  yeast;  it  acts  upon  dextrose  or  grape  sugar 


228  COMMON  CHEMICAL  PROCESSES 

(§  242),  breaking  up  the  dextrose  into  carbon  dioxide  and 
alcohol.  Since  dextrose  occurs  widely  in  fruits,  this  sort 
of  fermentation  is  very  common.  All 
sorts  of  alcoholic  stimulants  —  wines, 
whisky,  etc.  —  are  made  by  allowing 
something  to  become  fermented. 

Yeast  is  a  very  low  form  of  plant ; 
in  its  nature  it  is  somewhat  like  bac- 
teria.   Fig.  149  shows  a  few  bits  of 
yeast,  greatly  magnified.    It  is  found 
frequently  in  the   air,  from  which  it  easily  gets  into 
many  substances. 

Other  sorts  of  fermentation  are  common.  Apple  juice 
partly  ferments  and  becomes  cider;  then  another  fer- 
ment acts  upon  the  alcohol  in  the  cider,  changing  that  to 
an  acid,  so  that  the  liquid  becomes  vinegar.  The  sour- 
ing of  milk  is  a  process  of  fermentation. 

268.  Bread  Making.  —  Yeast  may  be  easily  made  to 
grow  till  it  forms  a  large  mass.  This  is  commonly  done 
by  many  cooks,  who  make  what  they  call  potato  yeast ; 
in  this  case  the  potato  serves  as  a  substance  in  which 
the  yeast  plant  may  grow.  Cakes  of  compressed  yeast 
may  be  bought  of  the  grocer ;  these  are  masses  which 
have  been  grown  for  the  purpose  in  large  quantities. 

Bread  is  made  of  flour  mixed  with  milk  or  water 
to  form  dough ;  yeast  is  added  to  "  raise  "  the  dough. 
Flour  is  largely  starch.  When  the  mass  is  put  in  a 
warm  place  the  yeast  acts  upon  the  starch,  changing 
it  to  dextrose;  this  is  further  acted  upon,  so  that  it 
ferments,  forming  alcohol  and  CO2.  The  CO2  cannot 


DISINFECTION  229 

escape  through  the  dough,  so  it  simply  forms  in  bub- 
bles, making  the  mass  "  light."  In  baking,  the  alcohol  is 
mostly  driven  out  and  the  heat  stops  any  further  action 
of  the  yeast. 

269.  Disinfection.  —  The  bacteria  of  which  we  have 
studied  are  very  numerous ;  there  are  also  many  differ- 
ent kinds.  They  are  too  small  to  be  seen  without  a 
strong  microscope,  except  in  masses  composed  of  many. 
Some  kinds  of  bacteria  are  harmless  and  some  are  even 
useful,  but  a  few  kinds  are  known  to  be  the  cause  of 
certain  diseases  in  animals  and  man.  These  kinds  are 
usually  given  off  in  some  quantity  from  persons  who  are 
ill  with  such  diseases ;  and  as  they  may  be  taken  into 
the  bodies  of  other  persons  and  there  cause  illness,  it  is 
important  to  try  to  kill  the  "germs." 

The  killing  of  these  bacteria  is  called  disinfection. 
Many  methods  are  used.  Heat  is  of  great  use,  as  a  tem- 
perature of  100°  C  (boiling  water)  will  destroy  all  com- 
mon forms  in  a  short  time.  All  dishes  and  cloths  used 
by  the  patient  should  be  carefully  boiled  in  water,  and 
papers  should  be  burned.  Fresh  air  in  the  sick  room  is 
important,  and  sunlight  kills  many  bacteria.  For  a  liquid 
disinfectant,  weak  solutions  of  carbolic  acid  or  of  some 
chlorides  are  good,  but  nothing  seems  to  equal  a  weak 
solution  of  corrosive  sublimate  in  water  (1  part  in  1000). 
After  all,  the  best  way  to  avoid  diseases  is  to  keep 
ourselves  clean  and  keep  our  general  health  at  the 
highest.  Many  disease  germs  doubtless  enter  the  body 
of  a  well  person  and  do  no  harm  because  of  his  strong, 
healthy  condition. 


230  COMMON  CHEMICAL  PROCESSES 

QUESTIONS 

1.  Define  combustion.    What  is  a  combustible?    In  common 
fuels,  what  elements  usually  burn  ?    What  substance  commonly 
supports  combustion  ?    Name  the  compounds  generally  formed  by 
combustion  of  fuels  in  air. 

2.  How,  in  general,  may  we  start  combustion?    How,  after 
being  started,  does  the  process  keep  itself  going  on  ?    By  what 
means  may  combustion  be  stopped  ? 

3.  What  is  an  explosion  ?    Why  is  an  exploding  mixture  able 
to  exert  so  great  force  ? 

4.  What  sort  of  substances  burn  with  a  flame  ?  How  do  solids 
burn  ?    Show  why  flames  are  seen  when  some  solids  (e.g.  wood) 
are  burned.    Explain  the  burning  of  such  substances  (e.g.  kero- 
sene, candles,  etc.)  as  furnish  light.    What  is  smoke  ? 

5.  What  is  meant  by  the  word  fire  ?    In  common  fires  explain 
each  of  these  :  the  heat,  light,  flame,  smoke,  and  ashes. 

6.  Explain  the  meaning  of  oxidation.    What  substances  com- 
monly form  oxides  in  this  way?    What  is  iron  rust?    Under 
what  conditions  is  it  usually  formed? 

7.  What  element  in  the  air  is  needed  by  animals  ?    What 
becomes  of  this  element  when  it  is  taken  into  the  lungs  ?    With 
what  does  it  unite  ?    Where  ?  What  does  this  process  supply  to 
the  body  ? 

8.  What  is  meant  by  decay  ?  What  sorts  of  substances  usually 
suffer  decay  ? 

9.  What  is  a  ferment  ?    Name  a  common  ferment.    When  a 
ferment  acts  upon  dextrose  what  is  formed?    How  is  vinegar 
made? 

10.  What  sort  of  a  substance  is  yeast  ?    Explain  the  action  of 
yeast  in  bread  making.    What  gas  is  formed,  and  what  is  its  use  ? 

11.  Explain  how  disease  may  be  given  from  one  person  to 
another.    What  is  meant  by  disinfection?    What  methods  are 
useful  ? 


INDEX 


[The  references  are  to  pages.] 


Absolute  cold,  86. 

Absorption  of  light  waves,  111, 
127,  128. 

Acids,  183,  184 ;  fatty,  208. 

Adhesion,  12. 

Air,  composition  of,  210 ;  com- 
pressed, 43;  dome,  39;  lique- 
fied, 86 ;  pump,  42. 

Alcohol,  190,  208. 

Alkalis,  184,  190,  208. 

Alloys,  188. 

Alternating  current,  158. 

Alum,  201. 

Aluminium,  201. 

Amalgam,  190. 

Ammonia,  85,  205. 

Ampere,  146. 

Aniline  dyes,  217. 

Annealing,  13. 

Anthracite,  215. 

Arc,  electric,  168 ;  lamp,  168 ;  of 
pendulum,  58. 

Armature,  of  dynamo,  156 ;  of 
motor,  162. 

Artificial,  cold,  84 ;  ice,  85. 

Atmospheric  pressure,  32,  33 ; 
effects  of,  34,  36-39. 

Atom,  176. 

Atomic  theory,  176. 


Bacteria,  227,  229. 
Balloons,  44,  194. 


Barometer,  36. 

Bases,  183,  184. 

Battery,    145 ;    uses  of   current, 

146. 

Bell,  electric,  166. 
Bell,  metal,  188. 
Brass,  188. 
Bread  making,  228. 
Brick,  212. 
Brittleness,  13. 
Bronze,  188. 
Buoyancy,  26,  27  ;  in  gases,  44. 

Calcium,  199;  compounds,  199. 

Camera,  122. 

Cane  sugar,  207. 

Capillarity,  17. 

Carbohydrates,  189,  219. 

Carbon,  195,  215,  226;  com- 
pounds, 189,  204 ;  dioxide,  195, 
204,  210 ;  monoxide,  205. 

Cars,  electric,  163. 

Cells,  dry,  143;  gravity,  143; 
.  kinds  of,  142 ;  voltaic,  140, 
141. 

Cellulose,  189,  206,  207,  213. 

Center,  of  gravity,  61,  52;  of 
mass,  52. 

Centigrade  thermometer,  70. 

Centrifugal  force,  55. 

Centripetal  force,  55. 

Charges,  electric,  133-137. 


231 


232 


INDEX 


Chemical,    action,   67,   177,  179; 

affinity,  176, 177  ;  changes,  108, 

171. 

Chemistry,  scope  of,  171. 
Chlorides,  186,  197. 
Chlorine,  197. 
Circuit,  143;  divided,  145. 
Cloth,  214. 
Clouds,  76. 

Coal,  195,  215,  216;  tar,  216,  217. 
Cohesion,  11. 
Coke,  216. 

Cold,  69;  absolute,  86;  by  vap- 
orizing, 85 ;  by  melting,  85. 
Color,   explanation  of,    124 ;    of 

light  waves,  127;    of  objects, 

127. 
Combination,  chemical,  177,  178, 

180. 
Combustion,   68,   179,  193,  195, 

221. 

Commutator,  158,  163. 
Compass,  153,  154. 
Composition,  chemical,   172;    of 

matter,  5,  8,  9. 

Compounds,  173,  174,  180,  202. 
Compressed  air,  43 ;  engine,  43. 
Compressibility,  15. 
Compression,  as  a  source  of  heat, 

67  ;  of  gases,  43. 
Condensation,  75. 
Condenser,  76. 
Conduction  of  heat,  79,  80. 
Conductors,    of   electricity,    130, 

134;  of  heat,  80. 
Contraction,  72,  73. 
Convection,  79,  80,  81. 
Copper,  200. 
Cotton,  214. 
Coulomb,  146. 


Crystallization,  16. 

Current,  alternating,  158 ;  direct, 

158;  electric,  140;  induced,  155; 

strength,  146 ;  uses  of,  146. 

Darkness,  112. 
Decay,  226. 

Decomposition,  172,  178,  227. 
Dextrose,  207,  227. 
Diffusion,  6. 
Dipping  needle,  153. 
Discharge,  137. 
Disinfection,  229. 
Distillation,  76. 
Divided  circuit,  145. 
Ductility,  14. 
Dynamite,  218. 

Dynamo,  130,  140,  156 ;  currents, 
157,  158;  kinds  of,  158. 

Ear,  93,  95 ;  trumpets,  103. 

Earthenware,  212. 

Echoes,  97,  98. 

Elasticity,  15. 

Electric,  cars,  163 ;  current,  140 ; 
discharge,  137  ;  lights,  167  ;  mo- 
tors, 162,  163. 

Electrical,  effects,  131 ;  energy, 
129,  130,  162;  potential,  131, 
132,  138. 

Electricity,  129 ;  charges  of,  133, 
134,  135,  136;  generation  of, 
140,  155 ;  static,  135. 

Electrolysis,  131. 

Electrolytic  effect,  131,  166. 

Electro-magnet,  149,  158. 

Electro-motive  force,  132;  in- 
duced, 155;  unit  of,  146. 

Electroplating,  166. 

Electrostatics,  133,  135. 


INDEX 


233 


Elements,  chemical,  173,  183; 
symbols  of,  182. 

Emery,  201. 

Energy,  68,  69;  definition  of,  3; 
forms  of,  5 ;  from  heat,  89 ;  ra- 
diant, 83;  transformation  of, 
86,  87. 

Engine,  compressed-air,  43 ;  gaso- 
line, 89;  naphtha,  89;  steam,  88. 

Equilibrium,  54. 

Essences,  190,  217. 

Ether,  82,  83,  110,  169. 

Evaporation,  75. 

Exciter  for  dynamo,  158. 

Expansion,  72,  73;  of  gases,  41. 

Explosion,  223. 

Explosives,  217. 

Eye,  121. 

Eyeglasses,  122. 

Fahrenheit  thermometer,  70. 

Falling  bodies,  56,  57. 

Fats,  208,  219. 

Fatty  acids,  208. 

Fermentation,  227. 

Fire,  221,  224;  engine,  39;  ex- 
tinguisher, 222. 

Flames,  223. 

Floating  bodies,  28 ;  law  of,  28. 

Fluids,  23;  pressure  in,  24,  25, 
30,  32. 

Focus,  of  lenses,  120, 122 ;  of  mir- 
rors, 115. 

Fog,  76. 

Foods,  218. 

Foot  pound,  60. 

Force,  4 ;  pump,  38. 

Forced  vibration,  98,  99. 

Friction  as  a  source  of  heat,  67. 

Fuels,  219. 


Fulcrum,  63. 
Fusion,  74. 

Gas,  6,   8;  compression  of,   15; 

expansion  of,  41 ;  illuminating, 

216. 

Gasoline  engine,  89. 
Gear  wheels,  65. 
German  silver,  188. 
Glass,  212. 
Glucose,  207. 
Gold,  9,  13,  200. 
Gravitation,  19 ;  law  of,  20. 
Gravity,  19,  20;  cell,  143;  center 

of,  52 ;  specific,  21,  29. 
Gun  cotton,  218. 
Gun  metal,  188. 
Gunpowder,  217,  218. 
Gypsum,  199. 

Hardness,  12,  13. 

Harmony,  105.  .    . 

Heat,  67;  effects  of,  71;  energy,  69; 

latent,  77;  mechanical  energy 

from,  87  ;  sources  of,  67  ;  theory 

of,  68 ;  transfer  of,  79. 
Horse  power,  146. 
Hydraulic,  jack,  31 ;  press,  30. 
Hydrocarbons,  188,  217. 
Hydrochloric  acid,  197. 
Hydrogen,  44,  183,  188,  189,  194, 

217,  219,  224. 
Hydrometer,  29. 

Ice,  artificial,  85. 
Illuminating  gas,  216. 
Illumination,   109;   intensity   of, 

115. 
Images,  formed  by  a  convex  lens, 

121, 


234 


INDEX 


Impenetrability,  10. 

Incandescent  lamp,  167. 

Inclined  plane,  65. 

Induction,  coil,  160 ;  coil,  use  of, 
161;  electrostatic,  136;  mag- 
netic, 160. 

Inertia,  17,  47,  50. 

Insulators,  130,  134. 

Iron,  198 ;  rust,  226. 

Kerosene,  188,  194,  217. 

Lamp,  arc,  168;  incandescent, 
167. 

Latent  heat,  77. 

Law,  natural,  3  ;  of  floating  bod- 
ies, 28 ;  of  gravitation,  20 ;  of 
machines,  62  ;  of  magnets,  152, 
153  ;  of  motion,  46  ;  of  reflec- 
tion, 113. 

Lead,  200. 

Lens,  120, 121, 122 ;  effects  of,  121, 
122  ;  focus  of,  120  ;  glass,  213. 

Lever,  61,  63. 

Lifting  pump,  37. 

Light,  108 ;  rays,  109. 

Lights,  electric,  167. 

Light  waves,  108,  109,  110;  color 
of,  124  ;  reflection  of,  112,  114  ; 
refraction  of,  117,  119 ;  velocity 
of,  110. 

Lightning,  138. 

Lime,  199. 

Line  of  direction,  53. 

Lines  of  magnetic  force,  148,  155, 
160. 

Liquefied  air,  86. 

Liquefying,  74. 

Liquid,  6,  7  ;  level,  25 ;  pressure, 
24,  25,  30. 


Locomotive,  air,  43;  steam,  89. 
Loudness,  102. 
Luminosity,  109. 

Machines,  60,  61,  64;  law  of,  62. 
Magnet,  147  ;  electro-,  149 ;  law 

of,  152  ;  permanent,  151 ;  poles 

of,  151. 
Magnetic,  action,  147, 152  ;  effect, 

131,  147  ;  field,  148 ;  force,  147 ; 

induction,  160 ;  lines  of  force, 

148 ;  poles,  151 ;  poles  of  the 

earth,  153. 

Magnetism  of  the  earth,  153. 
Malleability,  13. 
Mass,  51 ;  center  of,  52. 
Matches,  197. 
Matter,  2,  4  ;  composition  of,  8  ; 

properties  of,  10 ;  states  of,  5, 

6,7. 

Measurements,  electrical,  146. 
Mechanical  uses  of  heat  energy, 

89. 

Megaphone,  100. 
Melting,  74. 
Mercury,    9,    200 ;     compounds, 

200. 

Mercury  air  pump,  42. 
Metals,  80,   130,    184,   185,   188, 

200,  225. 
Microscope,  122. 
Mirror,  114. 

Mixtures,  173,  174,  180,  210. 
Molasses,  207. 
Molecular  theory,  9. 
Molecule,  8,  176,  180;  vibration 

of,  68. 

Momentum,  50. 
Motion,  laws  of,  46,  47,  48  ;  wave, 

91. 


INDEX 


235 


Motor,  electric,  162,  163. 
Musical,  instruments,  100  ;  tones, 
104,  105. 

Naphtha  engine,  89. 
Newton's  laws  of  motion,  46. 
Nitrates,  186,  195. 
Nitric  acid,  186. 
Nitrogen,  195,  210,  218. 
Nitroglycerin,  218. 
Noise,  102. 

Ohm,  146. 

Oils,  208. 

Opaque  bodies,  111. 

Ores,  185,  188. 

Overtones,  105. 

Oxidation,  225,  226. 

Oxides,  187,  188,  225. 

Oxygen,  189,  192,  210,  225,  226. 

Paper,  214. 

Parallel  arrangement,  145. 

Pendulum,  57,  58. 

Penumbra,  112. 

Percussion,  67. 

Permanent  magnet,  151,  158. 

Petroleum,  188,  217. 

Pewter,  188. 

Phosphorus,  196. 

Photographic  camera,  122. 

Physical  changes,  171. 

Physics,  scope  of,  2. 

Physiological  effects  of  electricity, 

131. 

Pitch,  103 ;  limiting,  104. 
Platinum,  14. 
Poles,  in  a  cell,  142  ;  magnetic, 

151;  of  the  earth,  153. 
Porcelain,  212. 


Pores,  9,  14. 

Porosity,  14. 

Potassium,  199;  compounds,  199. 

Potential,  electrical,  130,  132, 138. 

Power,  60;  electrical,  146  ;  horse, 
60. 

Pressure,  atmospheric,  32-39 ;  ef- 
fect of,  on  boiling  point,  74 ; 
fluid,  24  ;  liquid,  24  ;  transmis- 
sion of,  30. 

Prism,  119,  126. 

Prismatic  colors,  126. 

Propeller,  screw,  49. 

Proteids,  189,  219. 

Pulley,  61,  62. 

Pump,  air,  42 ;  force,  38 ;  lifting, 
37 ;  steam,  39. 

Push  button,  144. 

Quality  of  tones,  104,  106. 
Quartz,  201. 

Radiant  energy,  83. 

Radiation,  heat,  82,  83, 108 ;  light, 

108-110. 

Radicals,  183,  186. 
Rain,  76,  202. 
Rainbow,  127. 
Rays,  light,  109. 
Reaction,  48,  55. 
Reflection,   of  light  waves,   112, 

113 ;  of  sound  waves,  97,  98. 
Reflector,  114. 
Refraction,  117,   118;    cause  of, 

119;  use  of,  120,  121. 
Reservoir,  26. 
Resistance,  to  electric  current,  132, 

144 ;  unit  of,  146 ;  uses  of,  145. 
Resonance,  99. 
Resonators,  99,  100. 


236 


INDEX 


Retina,  121. 
Reverberation,  98. 

Salts,  185,  186,  188,  219. 

Saturation,  76,  189. 

Screw,  49,  64. 

Series  arrangement,  146. 

Shadows,  111. 

Shunts,  145. 

Sight,  far,  122 ;  near,  122. 

Silicon,  201. 

Silver,  200. 

Siphon,  39. 

Smoke,  7,  224. 

Soap,  208. 

Sodium,  199;  compounds,  199. 

Soil,  211. 

Solidifying,  74. 

Solids,  5,  7. 

Solutions,  189. 

Solvents,  189,  190. 

Sound,  definition  of,  92,  93 ;  loud- 
ness  of,  102 ;  musical,  105 ;  ori- 
gin of,  94. 

Sound  waves,  93,  164 ;  reflection 
of,  97;  transmission  of,  97; 
velocity  of,  97. 

Specific  gravity,  21,  22,  29. 

Spectrum,  126,  127. 

Speech,  106. 

Stability,  54.  - 

Starch,  189,  206,  207. 

Static  electricity,  135. 

Steam,  7,  8;  engine,  88;  loco- 
motive, 89;  turbine,  89. 

Steel,  198. 

Substances,  kinds  of,  172. 

Sugar,  16, 189 ;  cane,  207  ;  grape, 
207. 


Sulphates,  186,  204. 
Sulphides,  187,  188. 
Sulphur,  187,  196. 
Sulphuric  acid,  195,  204. 
Surface  level,  25. 
Symbols,  181. 
Sympathetic  vibration,  99. 

Telegraph,  165 ;  wireless,  169. 

Telephone,  164. 

Telescope,  123. 

Temperature,  69. 

Tenacity,  14. 

Theory,  atomic,  176;  molecular, 

9 ;  of  heat,  68. 
Thermal  effect,  131,  167. 
Thermometer,  69,  70. 
Tin,  200. 
Tinctures,  190. 
Tones,  101 ;  differences  in,  102 ; 

loudness  of,  102  ;  musical,  102, 

104, 105 ;  pitch  of,  103  ;  quality 

of,  104. 
Transformation    of    energy,    86, 

87. 

Transformer,  159. 
Translucent  bodies,  111. 
Transmission,   of  fluid  pressure, 

30;  of  sound  waves,  96. 
Transparency,  110. 
Turbine,  steam,  89. 
Type  metal,  188. 

Umbra,  112. 

Units  of  electrical  measurement, 
146. 

Vacuum,  33,  34. 
Vapor,  7. 


INDEX 


237 


Vaporization,  74. 

Velocity,  51 ;  of  light  waves,  110 ; 

of  sound  waves,  97. 
Vibrating  bodies,  94,  95. 
Vibration,  9,  91,  92,  94;  forced, 

98,    99;     of    the    ether,    110; 

rate  of,  92;  sympathetic,  98, 

99. 

Voice,  105. 
Volt,  146. 

Voltaic  cell,  130,  140. 
Volume,  27  ;  changes  of,  72,  73. 

Water,  composition  of,  202 ;  ex- 
pansion of,  73;  mineral,  202; 
use  of,  190;  vapor,  210. 

Watt,  146. 


Wave,  91 ;  length,  92,  103 ;  light, 
108, 109, 110;  motion,  91;  sound, 
93. 

Weather  changes,  36. 

Wedge,  65. 

Weight,  20. 

Welding,  12. 

Wheel  and  axle,  64. 

White  light,  125,  126. 

Winds,  82. 

Wireless  telegraphy,  169. 

Wood,  213. 

Work,  60 ;  electrical,  146. 

Yeast,  227. 

Zinc,  147,  200 ;  plate,  142. 


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